Distortion....Nothing Special

Sooooo, I came in early today to add another project to my line before school ended.
Nothing special..
Just a simple distortion circuit that I got from google.
I plan to actually go a little farther with it because the rhythm guitarist of my band sure can use one.
For now, it's just on and off, but I plan to add volume controls using a 100k pot and distortion controls using a 1k pot.
Hopefully, for being a simple distortion, it will sounds somewhat decent.

And hey! There's the guts! I just put dat shiit in a plastic temporary housing, but at least it looks better than my talkbox..

Personal Guitar Junk Issues =(

Those of you who didn't know, my car got broken into recently and a bunch of stuff got stolen including my beautiful Ernie Ball Musicman guitar =(

So I went back up to Washington to grab the guitar that me and my construction buddy have been working on for the last year. Unfortunately, he didn't finish it like I had hoped, so I had to do it myself. I had to install the pickups (which really sucks). I also brought down a guitar that I put together when I was a wee lad, but smashed after a show in high school, soooo I got to redo the pickups on that one too, hooorahhhh. But, they're both working, and I have some temp guitars before the insurance (hopefully) covers my loses so I can buy a new guitar.

Ghetto-ass talkbox

Just an update on my talkbox.
Tis all done, it's nothing too crazy, but it was a pain in the butt to put together especially when it came to the housing.

For the housing I just used an electrical box that I picked up at the hardware store.
To connect the rubber tube, I used a copper pipe joint and crazy-glued it onto the box.
From there I glued the speaker to the hole where the copper joint sits.
The circuit I used was a simple mono amp circuit.
Besides from the motorboating when the pot is not either on 1 or 10, it works like a charm.

Woop!

How to fix your headphones without buying a new pair.

Hello fellow audio sound makers,

In this post, I shall go over the process of replacing the eighth inch adapter on the headphones. Almost all studio monitoring headphones are connected through three wires, tip, ring, and sleeve; or ground, left, and right. Most headphones that go bad in one ear is caused by wear and tear on the eighth inch jack. All that simply needs to be done is replace the jack with a new one. Here is the process of doing so:

The first step is to break down the rubber or plastic bearing on the outside of the jack, to get to the actual connection of the wires to the jack. Then.....

Snip Snip. Cut that little guy off, exposing the wires.

All headphone wire is wrapped with magnet wire. The magnet wire is wrapped with enamel in the wire. The enamel is not a conductor, therefore it must be separated from the wire to make the wire conduct. The magnet wire is used in headphone cables for the purpose of providing insulation while keeping the diameter of the wire small. Notice Sudhu doing a splendid job of trying to take that enamel off of the wires.

Once the enamel is scrapped off and an electrical signal can be established, a new eighth inch cable can be soldered on.

PNP vs NPN video: Breaks down the transistors in a very simple instructional tutorial

Hey all!

I found this video to be extremely helpful in describing the difference between PNP and NPN transistors.

Takeaway:

NPN uses a positive signal to allow current to flow through the circuit
PNP uses a ground signal to complete the circuit to allow current to flow through the circuit

http://www.youtube.com/watch?v=F-Cv7CMHoGM

JFET vs BJT

In a nutshell, JFETs operate off of voltage while BJTs (Bipolar Junction Transistors) operate of off current.  The JFET is much more sensitive making it more accurate, however, is also more susceptible to damage from static electricity.

Link here: http://www.electronics-tutorials.ws/transistor/tran_5.html

Great info on 12V Zener Diodes

Here's a quick screenshot I thought was very useful...

The full article is here:

http://www.evilmadscientist.com/2012/basics-introduction-to-zener-diodes/

4th Connection on XLR cable (Accidentally deleted this post from earlier)

I posted this earlier and accidentally deleted it while editing a draft.

For those of you wondering what the 4th connection on an XLR cable is, it's called a "shield ground" in where the ground is connected to the entire metal casing of the cable.  It is usually not recommended to use this method being that once the cable is plugged into a system that has an alternate ground it may create an "earth loop."

Here's what one blogger had to say:

Why not to connect.

If you use the fourth lug you connect the metal parts of the connector to your “sound system earth”.  If the metal parts of connector then makes contact with something that has a different earth eg. lighting truss, metal parts of the stage etc. you have an earth loop.  

Electret Microphones need phantom power?

Today I tested my circuit using an apogee duet as my interface and Pro Tools for audio output.  During my test, I connected the circuit using a male XLR into input 1 on my apogee and surprisingly received a signal (Previous attempts didn't even register noise).  I was getting somewhere.

I continued to test by tapping on the capsule end of the circuit to see if it would react accordingly and adjust DB levels in Pro Tools.  It did just that, however, the DBv level was extremely low.  Realizing this, I tested with Phantom Power and the result was a much higher DBv level.  Confusing?  Yea, a little bit.

Electret Condenser Microphones typically do not need phantom power being that they hold a small charge (capacitance) between the fixed and moving plates of the capsule.  However, the phantom power isn't used for the microphone diaphragm itself, but rather, it is used to power the JFET.  As stated in my post about JFETs, these transistors are used to buffer voltage and control the flow of voltage throughout the circuit.

Alice Microphone 2N 4416 JFET Spec Sheet

This post relates to my last post on ECMs and JFETs.  It is the spec sheet for the JFET used in the Alice Microphone Schematic

http://www.colorado.edu/physics/phys3330/phys3330_fa11/pdfdocs/2N4416a.pdf

Why Electret Condenser Microphones use JFET's

Hey all,

So during my construction of the Alice Microphone, I put together the circuit and was able to pass signal (noise) but couldn't capture audible noise such as speech or intelligible audio.  I figured doing a little research on the parts would help.  I've started with the main part of the circuit, the JFET (Junction Field Effect Transistor).

After doing some reading, the JFET is the most commonly used transistor in Electret Condenser Microphones (ECM's) being that it serves as a high impedance buffer/amplifier for Analog to Digital Conversion at the input stage.  As most of us know, an ECM holds a pre-charged voltage in its capsule.  When the sound pressure enters the capsule, the moving plate will adjust accordingly, thus changing the capacitance and output voltage.

Below you will find some of the benefits of a JFET when it comes to ADC.

Here is the link to the site http://www.ti.com/lit/wp/snaa114/snaa114.pdf

Bill of Materials for Echo Base

I found all the parts I needed for this pedal. Most are available in the classroom, I found the rest of the components such as the PCB board, multiple IC's and specific transistors on Small Bear. I'm going to order the parts this week and start assembling it on a bread board next week.

I made three different lists. The first one is a list of all the basic components I need, and it mostly comprised of the components I need to buy. The second page is reserved for all the resistors I need, while the third is for the capacitors.

 

PT2399

The PT2399 is a delay/echo integrated circuit by Princeton Technologies. Although digital in nature, it is frequently used to emulate the sound of analogue (BBD- based) delay circuits by means of additional filtering. Its being found more often lately in a lot of DIY delay pedals.

http://www.diyaudiocircuits.com/tutorials/pt2399-digital-delay-analog-echo/

This link has a lot of useful information regarding this chip, and also has a link to the full and unabridged spec sheet, which is surprisingly hard to find online.

Echo Base Delay

The Echo Base is an analog-voiced delay with modulation, designed by Ian Maltby. It uses a PT2399 delay chip to create a sound similar to an analog delay,  but not as noisy and without the hassle of building a real analog delay. The modulation allows you to wobble the pitch of the delay around. This allows for tape-style “wow and flutter”, vibrato, and chorus sounds in addition to the delays.

The “tails” feature allows you to turn the delay off while letting the last few echoes die off, rather than cut off abruptly. This feature can be toggled on or off with a switch. Lots of useful modifications are possible with the Echo Base, and are shown in detail in the accompanying documentation.

The documentation and the PCB circuit for this pedal are available here:

http://musicpcb.com/pcbs/echo-base-delay/

This pedal seemed quite popular on all the forums as an accurate imitation of an analog delay, but not as complex and temperamental to build. I would reccomend it to anyone looking for a solid DIY delay pedal that comes with proper instructions. I also happened to listen to some of the sound samples, and it actually sounded pretty good. Ordering the PCB and all the other parts I need this week. If anyone has pedal housing unit or a cool looking box they can spare, let me know.

Midterm Schematic

This is the circuit I built for the midterm. Sorry about the blurry picture, that's the best my phone can do. I was initially going to add a delay section to this existing tube screamer circuit, but I've now decided to build a delay separately. This way I won't have any compatibility issues trying to match two different circuits, and can also add other touches to my delay such as modulation. I still want to finish this pedal later on as it wasn't too difficult and seemed like it would sound good.

Tape Bias, Homey

A little description thingamajig on tape bias!

Bias is a unique signal that is applied when recording on tape. There are two types of bias, AC and DC. Back in the good ol' days, DC was where it was at. The first recorders simply added the raw audio signal to the electromagnet in the record head. In other words, it is the direct current being added to the recorded audio signal. The problem with DC bias is that it results in poor low-frequency response and high distortion.

Later on, when people got their shiit together, AC bias was invented. This was a new and improved bias that resulted in less noise. AC is the addition of an inaudible high frequency signal to the recorded audio signal. This signal usually ranges between 40kHz - 150kHz. Bias increases the audio quality by pushing the signal into the linear zone of the tape's transfer function.

Vigen BBD project week 7 part 1

Hello All,
I have finally got my circuit to work and now I can begin to add the filter sections and tweak the circuit to get a better sound.  It currently is extremely noisy since their is no low pass filter and the voltage bias on the BBD and clock have not been optimized.  Setting the bias is not easy and requires some time and patience in setting up the circuit.  It is also a very fine tuning process since the BBD chip wants to see a very narrow range of voltage values.  This is why most of these types of pedals use a compander section to compress the signal voltage to usable levels and then expanding the echos upon reaching the output stage.  More details to follow.  I found a great website for DIY pedal building that provided a nice schematic, parts list and even a circuit trace for a functioning BBD-style delay pedal!  I will be analyzing this schematic and then continuing my circuit bread board.  Hopefully, I will be able to tweak the design to make my own version and to create an awesome analog delay pedal.  Here is the link to the resources and I will be posting some updates of my design here soon.

AD-3208
http://www.generalguitargadgets.com/projects/20-modulationecho/48-ad-3208

Edward

Vigen BBD project Week 6

Hello All,

So my BBD project is not working quite yet. There are many factors at play here, not least of which is the quality (or lack thereof) with regards to the bread boards. Here is a listing of the issues encountered so far:

Bread Board Issues

Bread boards can be notorious for having to get the parts seated right. This just means that as more parts are added to the circuit, you need to check and re-check every part to make sure it sits properly in the bread board.

BBD driver issues

I unfortunately, have not been able to get my BBD chip to work yet. Part of the problem here is the biasing needed for driving these chips. Whereas opamps, transistors and many other components have to be biased simple ratios like ½, BBD chips need a 14/15 ratio. In the older model chips, like the MN3005, MN3007 and so on, which ran on +15V, the newer chips like the MN3205, MN3207 and the one I am using, the BL3208, all run on +9V. I have still not isolated the proper voltage divider with simple resistors and in looking at schematics for most of these circuits, they utilize a carefully tuned trimpot resistor to bias the chip. Hopefully I will have this figured out by the end of the next class period.

Circuit Integration

Beyond these issues, most of the schematics available online for analog delay pedals are not correct or missing certain components. I have been able to find limited resources in regards to setting up the clock source within a circuit that is leveled beyond the simple schematic in the Stompbox cookbook. Datasheets for the clock driver are also very vague in regards to the test circuit, ie one is not provided, and I have been having to use the Stompbox cookbook as a reference.

I am at this point attempting to simply build the basic sections for the pedal such as preamp buffer, BBD section and then the pre and post filters. I have a working preamp buffer and I am currently working on the BBD section. I have attached photos of each section as well as the schematics of each section as built so far in a subsequent post. If I cannot successfully get the BBD to work by the end of class week 7, I will probably have to switch over to building my Alice microphone for time purposes and I so I have a completed project at the end of this class. The knowledge gained on this project has been very helpful but it is definitely not an easy project at all. There is reason why you pay high prices for analog BBD delays.

Edward

Here are the pictures of my circuit as well.

Hello fellow classmates and electricians,

Before you I have the schematic for the new distortion mod that I created in class. It is much different than the original schematic I posted earlier.  

With this circuit, I have created a multi-use circuit that does more than distortion. More details about it tonight....

Update on some shiii..

Just an update on what I'm doin.
I came in on Wednesday to work on some extra stuff, to add to my tape delay I built a preamp (left board) in hopes to replace that big honkin' preamp off of the tape player. It's just a simple circuit that our teach suggested I use. The right board is just the mixer that I made last week.
Welp..that's pretty much it!
Huzzah

Greetings fellow audio electronics-men,

This week, I have finally received my microphone in the mail today. This should be a fun build I can't wait to get started and to keep bugging Sudhu to help me out!

Self Oscillation circuit in the pedals

Hello All,
Here is a link to a thread I found had a couple of tidbits of fun information.

Self Oscillation Circuit
http://experimentalistsanonymous.com/board/index.php?topic=152.0;wap2

Obviously many delay pedals such as the MXR M169 Carbon Copy can be set to do this with no modifications.  However, it is quite simple to set this up in an existing circuit and the great nugget of information is the part about how to set the bias resistor to create feedback at shorter delay times and/or completely eliminate it.  Pretty simple stuff but just another thought of interest for understanding all the components going into some of these designs.

Peace,
Edward

Just some shiiit (not a resourse)

Howdy, just postin what's up with one of my projects. If you somehow didn't know what I'm doing, I'm doing an analog tape delay out of some old cassette players; a simple mechanism, but with some annoying little obstacles that shouldn't be too hard to tackle.. Anyways, here are some close-ups on what's been done so far:

As you can see from the first picture, so far we have our main player which is acting as our motor to move the tape with the record/play head removed for convenience to the far left of the loop. The second play head (6:00 of tape loop) will be acting as the "echo" head. What I am hoping to do is possibly make my own preamp to use in place of the large circuit board shown in the top left of picture 2 so that way it could use less real estate in the enclosure when that is made. I am planning on creating a simple mixer as well so some extra parameters can be controlled. The next step for now is figuring out how to create a line input into the delay so an instrument can be plugged into it.

Anyways, that's whats up with this. Woop!

Tape Delay Circuitry

Using the web as a resource! This thread outlines exactly the obstacle Colton is faced with in building a tape delay:
http://www.electro-tech-online.com/electronic-projects-design-ideas-reviews/111155-cassette-recorder-circuit-guitar-effects.html


This would be a great IC to use, if we could find a place to sell us one...
UTC TA7668:
www.bucek.name/pdf/ta7668.pdf‎

Equivalent ICs and Spec Sheets!

Since a bunch of you are working on older pedal circuits that used the 4558 dual op amp IC, I poked around a bit to see if we could replace that harder to find chip with something we already have on hand, at least for breadboarding and prototyping.

I found this thread by doing a google search for "4558 equivalent".
http://music-electronics-forum.com/t19942/

I thought this would be a good chance for us to get better and understanding and using spec sheets, so check these two out and let's talk about them next week in class:

TL072:
www.ti.com/lit/ds/symlink/tl071a.pdf

MC4558:
www.st.com/web/en/resource/technical/.../datasheet/CD00000478.pdf‎

and another, just for fun, RC4558:
www.ti.com/lit/ds/symlink/rc4558.pdf

Keeping it Simple

Hey all,

I've decided to go simple and build the Lava-Rim 2 distortion pedal off of DIY Stompboxes.  I figured since I just entered the world of guitars a mere 10 days ago, keeping it simple would be the most logical way to go.

So, here is the link to the schematic if anyone else is interested.
http://www.diystompboxes.com/pedals/lava2.GIF 

I'll be doing research on the parts to figure out the type of transistor used and why it is built in this fashion.

Video on Transistors (Transform + Resistor = Transistor)

Hey all

Here's a short YouTube video clip on FETs, also known as Field Effect Transistors.  Short and has a bit of humor int it.  When It gets to the PN portion and the flow of electrons, I got a bit bored but the information/jargon helped me understand function when researching further. Enjoy!

http://www.youtube.com/watch?v=CLLcRRBph90

Useful Website for Schematics

I came across this website a few days ago.  It has schematics for all kinds of guitar pedals which could be useful for this class or future projects.

http://www.muzique.com/schem/index.html

Personally I'm interested in giving the Ibanez Tube Screamer a shot as it doesn't look as complex as I thought it would.

Acoustic Delay Pedal

Hey guys,

I'm finally linked in to this blog. So, I'm joining you all to build delay pedals being that I received an acoustic/electric guitar for my birthday just a week ago. 

With that said. I'm currently on this site...

http://www.mammothelectronics.com/EA-Tremolo-GuitarPCB-com-Pedal-Kit-p/kit-gpcb-eatrem.htm

Any thoughts?

Possible Project?

Howdy howdy.

I've always been a fan of the talkbox with my guitar. I used to have one but unfortunately I had to sell it awhile back. If you by any chance don't know what it is, it's a pedal basically with a built-in amplifier and EQ that's connected to a rubber tube that lets you control the sound with mouthing out words. That classic Peter Frampton sound.

Is anyone interested in possibly doing one of these? It sounds like it's something that could be simple.
I found the schematic for the model talkbox I used to own, take a looksie..

Hello All,
Here are two more analog delays from popular manufacturers that highlights the similarities in the design of a simple BBD analog delay pedal.  Good reference point and we could potentially all build a different circuit.  Thoughts welcomed.  I will also be posting more information later tomorrow for design thoughts, tips and some signal flow traces.

Edward

Boss DM-2
http://www.hobby-hour.com/electronics/s/dm2-delay.php


Ibanez AD-9
http://experimentalistsanonymous.com/diy/Schematics/Delay%20Echo%20and%20Samplers/Ibanez%20AD9.pdf

MXR Distortion+

Hello fellow classmates!

I have found a cool distortion pedal schematic online that I am going to build in class. It is an MXR distortion pedal from the 1970's. What's good about this pedal is first off, we have all the parts needed to build this at school in the locker. The second interesting thing about this pedal is it has a dual power source option, from either a battery or a plug in right from the wall. This is possible by the two power inputs being connected to different parts on the quarter inch jack. The AC power supply is connected to the battery being connected and activated through the ring of the jack, and the AC power supply being connected and activated through the tip of the jack. This way, you can have two options to powering up the pedal.

http://distortionpedals.org/wp-content/uploads/2012/10/mxr-distortion.gif
Joe

BBD Circuit Explanation

Hello All,

Here is a breakdown of the various stages of an analog BBD style delay circuit. This is overly simplified and is more concerned with the various stages and the components needed as explained in the “Delay” section of The Stompbox Cookbook. I have provided a visual reference with color coding to make the circuit a little more clear.

Input Buffer/A1-Salmon

This is the initial stage of the pedal and is run by a single op-amp. This section is very straightforward and can be used to increase gain, provide some filtering if desired and is the first section encountered by the input signal. It is biased by Bias Buffer/A3.

Pre-Filter/A2-Green

This is the pre-BBD filter section to remove clock noise. This section can get very extensive and complicated depending on how much filtering is desired and/or necessary. With with very long delays, such as those in excess of 400ms, the clock rate ends up in the audible spectrum and must be removed with a very steep filter or the resultant delays will be “noisy”. This section is also biased by the Bias Buffer/A3.

Bias Buffer/A3-Sepla

This section is used to provide a biasing voltage to the input and pre-filter op amp stages. This section also feeds the post filter with the same voltage at its non-inverting input.

Post-Filter/A4-Light Blue

This section, much like A2, is for post filtering of the clock signal from the audio signal. This section is very crucial and be quite extensive, as in the case of the Electro Harmonix Memory Man, with multiple filtering segments running through this section of the output stage. This section also provides the output buffer for feeding the next pedal or amp in your signal chain.

BBD IC Voltage Feeds-Olive-Sepla

The Bucket Brigade Delay is used to provide the delay in milliseconds for the operation of the chip. Here the chip can consist of up to 4096 capaciators that are alternately triggered to release voltage in an on/off fashion. The rate of this action is determined by the BBD Driver which is a separate clock source IC that feeds Clock 1 and Clock 2 inputs on the BBD IC. The BBD IC has many input and output paths that are similar in function on all BBD ICs and it is biased by its own trimpot.

BBD Driver-Light Orange

The BBD Driver is used to supply the clock 1 and 2 voltages to the BBD IC and to control the rate of delay. A trim pot is often used here to alter the delay time and this circuit is fairly straightforward in its design.

There is definitely more to any actual pedal design such as the addition of a compander that compresses the input signal to the BBD IC and then expands it upon output to provide consistency in the delay repeats and to provide optimal input/output signal. This section is not necessary to design a functioning pedal but in pedals that provide longer delay times, it may be desirable if signal fidelity is desired.

Ecthing circuit boards

Here are a few different ways to etch your own circuit boards:

This is probably the cheapest, the laser printer toner transfer. All you need is the etchant chemical and a copper clad board (I've also done this using a sharpie to draw the resist onto the copper clad board):
http://www.instructables.com/id/PCB-etching-using-laser-printer/

Jameco sells a kit for $46 to do photo resist etching, which is by far the best way to make really professional boards. Check out the video on their site explaining the process:
http://www.jameco.com/Jameco/PressRoom/makeoneetch.html

Making a Tape Delay while waiting for parts

For those of you interested in making a tape delay while we're waiting for our parts to arrive here's a nice explanation of tape delay from the Stompbox Cookbook:




Here's a picture of the one I built in high school with a hacked boombox and two extra tape heads:

Below are some other examples of homemade tape delays. You can find lots more with a quick google search.

We'll make a much better one with our fantastic knowledge of electronics and mixing circuits...

Makezine two walkman tape delay:
http://makezine.com/2008/04/30/diy-analog-tape-delay-mac/

Hacked tape deck delay:
http://hackaday.com/2010/12/20/tape-delay-made-from-recycled-cassette-decks/

BBD Component Substitution List

Hello All,
Here is a list of substitute parts for the impossible to obtain parts for the Delay pedals.  These are all available from Small Bear Electronics and would require minimal reconfiguration of the circuits.  Let me know what you all think and then I can order the amount of parts that we would need.  Should be about $20 a person to purchase the components that we can't raid from the school supply (resistors, capacitors, op-amps, etc). 

BL3208 (direct substitute for the MN3208 which is a 2048 stage BBD driver)
http://pdf1.alldatasheet.com/datasheet-pdf/view/164433/BELLING/BL3208.html

BL3102 (direct substitute for the MN3102 which is a BBD Clock source)
http://pdf1.alldatasheet.com/datasheet-pdf/view/164431/BELLING/BL3102.html

We will need to use two of the 3208s to simulate an MN3205 or MN3005 which is a 4096 stage BBD.  There is a diagram for this type of configuration on the small bear electronics website.
https://www.smallbearelec.com/HowTos/MN3005/3005_to_2x3008_SCH.pdf


Edward

Component Datasheets

Hello All,
Here are the datasheets for the most popular BBD, Clock source, Operational Amplifier and Compander IC's that I have seen in the delay schematics.  I will post more to this as I find them as a resource and if anyone finds any others, feel free to post them here.

BBD-MN3005
http://pdf1.alldatasheet.com/datasheet-pdf/view/87983/PANASONIC/MN3005.html

Clock Generator-MN3101
http://www.alldatasheet.com/datasheet-pdf/pdf/14238/PANASONIC/MN3101.html

Clock Generator-MN3102
http://pdf1.alldatasheet.com/datasheet-pdf/view/158293/PANASONIC/MN3102.html

Compander-NE570
http://pdf1.alldatasheet.com/datasheet-pdf/view/82893/PHILIPS/NE570.html

Compander-NE571
http://pdf1.alldatasheet.com/datasheet-pdf/view/18878/PHILIPS/NE571.html

Op Amp-4558
http://pdf1.alldatasheet.com/datasheet-pdf/view/115033/ETC1/4558.html

Edward

Various Analog Delay Schematics

Hello All,
Here are a couple of schematics for various analog delays from popular manufacturers.  I am posting these for comparison and analysis purposes. 

Boss DM-2
http://www.hobby-hour.com/electronics/s/dm2-delay.php


Ibanez AD-9
http://www.dirk-hendrik.com/Ibanez_ad9_analog_delay.pdf

We're going to use this blog as a place to share info (and progress) on building a bucket brigade delay.

Colt suggested starting with the Echo-o-matic in the Stompbox Cookbook (posted on my dropbox)
There's a great explanation of delay effects on page 144. Vigen is going to post his interpretation of the circuit (with colors) this week. Understanding the various stages will be helpful in interpreting any other bucket brigade delays we might try to build.

Vigen is in favor of building the Deluxe Memory Man from a schematic we found here:

And some great info on bucket brigades here:
http://www.gearslutz.com/board/geekslutz-forum/615501-bucket-brigade-delay-chip.html

Echoplex Tape Delay

http://www.loopers-delight.com/tools/oldechoplex/tape_echoplex_schem.html