Was about PCM - now about MPU trigger signals

droffset · Last edited by droffset on Tue Aug 18, 2009 3:19 pm; edited 1 time in total

Looking at some older music keyboards I'm modifying and seeing that all of the rhythm sounds boast about being done via pulse code modulation, so did a bit of digging.

It's a way to sample and encode audio data by slicing it up into pulse amplitudes.

Maybe this info is useful to people,

http://cbdd.wsu.edu/kewlcontent/cdoutput/TR502/page13.htm
http://www.tpub.com/neets/book12/49l.htm

http://en.wikipedia.org/wiki/Digital_audio

http://parts.digikey.ca/1/3/audio-codec-pcm-ic

Ideally I'd be able to hack into the keyboard's data bus and trigger percussion pulses. (For those keyboards that don't have actual percussion touch buttons. Maybe can use something like a datalogger to figure out the necessary pulses, or puck around with the clock, not sure.

Also trying to think of a usage for Arduino + Lunetta.

magman · Joined: Feb 04, 2009 Posts: 363 Location: Liverpool, UK

PCM is the industry standard way of encoding analogue signals into digital. It is used on telephone systems, CD players, WAV files, digital samplers and Digital Audio Workstations. It is the normal output of a Analogue to Digital converter before any fancy processing is done on the data stream (such as MP3, etc.). It effectively means digital sampling of an analogue signal at a given sampling rate (CD being 16 bit samples at 44.1kHz).

Advertising the fact that PCM was used to save rhythm sounds was marketeers picking up on one of the first technical buzzwords for digital audio. I wouldn't be surprised if the quality isn't that great either, given early D/A converters (8-bit I bet).

I doubt that any control of the rhythm samples can be made using PCM, unless there is some other embedded coding.

Regards

Magman

droffset · Posted: Tue Aug 18, 2009 2:34 pm Post subject:

Thanks for that, it looks like I was confusing the PCM audio output with the command signal that a button sends to the microcontroller to play a given sound.

On this keyboard there's a 40 pin microprocessor and a 24 pin Yahama sound chip. Can track which pins send snare, bass drum, etc from mpu to sound chip and can see on a scope that the pulses patterns change slightly when each signal command is given. I think that's what I want to be able to trigger on command, since the rhythm patterns are presets and don't come with drum pad buttons. Gate control of those sounds would be pretty fun.

Also found the audio output pins that get mixed elsewhere on the pcb, so individual audio out jacks seem straightforward.

Somewhere in that command steram must also be ADSR commands to mess with hopefully.

Will do another post with pics that explains what I've found a bit more in depth when i get to my own computer.