29 Nov 2005 svo   » (Master)

Hi everyone,

it's been a while since my last post. I'm currently off the track with Akipaki, even though shaft encoders are complete. The biggest problem is weakness of motors and it kind of embarrasses me because I can't find better motors. So I put it away for a while until better times (except that the main board is actually very useful for other experimental things - I/O and stuff).

I'm keeping myself busy with a S/PDIF decoder project. As trivial as it sounds, digital audio is not really a piece of cake (not that analog one is, either). At first I tried to build the decoder from scratch. First experiment was just a signal detector and a PLL loop to recover clock from Biphase Mark Coded signal. It was only moderately succesful, because I knew little about PLL's and the nature of incoming signal. Although trashed, it was still a great experiment that gave me good insight into the nature of PLL loops and now I'm not afraid of them. Another bonus was my first successful board with TSSOP16, 0.65mm pitch and an imported bitmap - before I only did 0.8 for TQFP44.

The next step was designing a new board, still in discrete logic, accounting for hard learned knowledge. This design would have proper preamble detection, PLL loop with divide-by-64 feedback, all the decoding circuitry - all there is to output a stream of recovered sound bits. But just as I was contemplating adding 15th package to my circuit, I found out that most decent DAC's require data MSB first. SPDIF data comes LSB first. That would make even more bricks that take real estate. What a bummer.. I quit.

Currently I'm designing a new board, now using integrated A/V processor from Analog Devices, ADAV801. It's a very cute little chip that can do everything one might need to route sounds in all directions, SPDIF in and out, including quality sample rate conversion and two single-ended integrated DAC's (which I'm pessimistic about). Simultaneously I ordered samples for AD1955 dedicated DAC which seems to be far more promising quality-wise, especially if put on a different board. And of course the whole thing is controlled by an ATmega8L, you can't go without a controller these days.

Wish me luck, routing PC audio to another corner of my room turned into a pretty ambitious project. I'll keep you posted.

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