Obsolesence Management, what a P.I.T.A.!

Well, I'm going to let a bit out of the bag on Secret Project #1 here,  but that's how things go, as I'm pretty darned excited on this one.  Secret Project #1 is going to be a highly modified variation of the speech/sound boards used through the System 6/7 era in a single integrated board, as opposed to separate speech/sound boards.  Ok, no so secret anymore.   But that's ok, I have a LOT more behind the scenes on this project that you're not going to see for quite some time.  Lets just say that I am not making a simple reproduction on this one by any means, so this should build up a bit of anticipation. I am looking to make an overall better peforming board.

Part of the problem I was having was the voice decoder chip on the speech board.  There are two different speech boards out there, which are interchangeable, but have a few differences between them.  Mostly though the differences are centered around the speech decoder chip used,  one of them uses a Harris 55516, the other a Motorola 3417, and their difference in ancillary components.   These chips are known as CVSD decoders.  For a good idea how they work there is a great YouTube video by a woman named Jerri Ellsworth that explains it very well.

The CVSD chips used on the original speech boards haven't been made in quite a long time.  Typical, eh?  And this was a huge stumbling block as I worked on my design.  If I can't reliably get a chip to decode the speech,  I can't do a speech board, and it becomes a dead project.  It also doesn't help that there wasn't that much info out there about CVSD speech encoding and decoding.  In fact, it was the only part that really kept me from seriously proceeding with the project. Though to be honest I haven't looked much into the 1408 DAC chip yet.  So who knows what's going to come up when I get to that part of the circuit.

But lets get back on track here. I finally found a viable option for this. Jerri, in her video, used an FPGA which sounded great, and is an awesome bit of engineering. However I am looking for an easier design for production purposes.  Well, I finally found one in a current manufacture CVSD chip that can use the 3-bit algoritim used in the older 55516 chip. The 55516's CAN Be found, but at $25 per chip they just aren't cost effective and its an uncertain future supply as it's mostly just old stock that happens to be sitting in inventories.  As far as I can tell, the other chip I found is still in manufacture and is about 50% cheaper.  

What I'll probably end up doing though is starting off with just a speech board prototype that is compatible with the original sound boards, but using the alternate chip.  After all, I need to make sure to get the op-amp circuits built up properly as well.   But it's exciting stuff knowing that I can move along some more now.  If it works, I may offer it as a stand-alone product,  but it's not the main goal of this design exercise.