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Adding S/PDIF output to PCM56P?
Adding S/PDIF output to PCM56P?
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Old 1st April 2014, 03:08 AM   #11
abraxalito is offline abraxalito  United Kingdom
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You'll almost certainly require some additional chips, perhaps even a CPLD. From a quick look at this there will be two challenges - the first is getting the right frequency for the OMCK pin of the 8406, Second, and perhaps the greater of the two - working out how to get the correct timing of the framing (WS in I2S parlance) signal.

Fig7 in the PCM56 DS shows that for each audio sample output, there needs to be a cycling of the LE pin. However one chip is doing double duty so there are two cycles of the LE pin per audio frame. None of the standard digital audio formats have two cycles of the framing pulse per audio frame, they have just one. So somewhere you'll need a divide by 2 function i.e. a flip-flop. You'll need to be a logic designer to get this to work.
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Old 1st April 2014, 05:03 AM   #12
Jidis is offline Jidis  United States
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Thanks abraxalito. I guess that's out then.

I didn't realize there were that many combinations being passed around between some of the common chips, and had probably given the 8406 too much credit. I thought it could work with most of them on its own and it was just a matter of setting the control pins for it.

Thanks Anyhow,

George
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Old 1st April 2014, 05:26 AM   #13
abraxalito is offline abraxalito  United Kingdom
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From a discussion some months ago, there was an ancient CD player which used a Sony digital filter to drive a PCM56P and that Sony chip also has an S/PDIF output. If your unit was lucky enough to use that same Sony chip you'd have a simple solution.
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Old 1st April 2014, 06:12 AM   #14
Jidis is offline Jidis  United States
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Unfortunately, mine are being fed directly from a big DSP chip. Got a feeling it probably could have handled that task as well, but only if the designers intended it to.

George

PS- I saw a few things when I was looking which also used a single PCM56 for both channels @44.1/16bit. I'd have thought that was more common and that the popular SPDIF transmitters would know what to do with it.
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Old 1st April 2014, 06:23 AM   #15
abraxalito is offline abraxalito  United Kingdom
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Multibit DAC chips these days are definitely in the minority since S-D DAC chips are much cheaper to produce and suit digital foundry processes. I'd be interested to know of any relatively modern kit using PCM56 or similar.
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Old 1st April 2014, 10:50 AM   #16
googlyone is offline googlyone  Australia
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Adding S/PDIF output to PCM56P?
Agree with the assessment provided by Abraxalito. The input / implementation for the CS8406 is super simple provided you have an I2S interface to drive it. If you want one I can post it for you - the PCB is 2 by 3 inches. The implementation only remains simple if you have a standard I2S stream including LRCLK, MCLK and DOUT.

The PCM56 data stream would require some modest massaging to get it close to this form. Quite possibly the easiest way to achieve this would be using a micro like a PIC, as this would allow you to clock in the audio data using one serial I/O port and pump it out as SPI on another port - which could fairly conveniently drive an SPI to SPDIF output.

You would need to be quite careful with clock rates and the implementation in the PIC, particularly relating to the choice of crystal frequencies, but I think this would be "doable".

As an aside, have you been able to dig into what other interfaces and data ports exist on the DSP in the system?

There does come a point however where you are easiest cutting your losses...
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Old 1st April 2014, 12:52 PM   #17
macboy is offline macboy  Canada
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Quote:
Originally Posted by Jidis View Post
Unfortunately, mine are being fed directly from a big DSP chip. Got a feeling it probably could have handled that task as well, but only if the designers intended it to.

George

PS- I saw a few things when I was looking which also used a single PCM56 for both channels @44.1/16bit. I'd have thought that was more common and that the popular SPDIF transmitters would know what to do with it.
No it is definitely not common to use a single DAC for two channels. This was obviously a cost saving exercise from the early days of digital audio when a high quality (for the time) DAC like the PCM56 was quite expensive.

I have to admit that when I first replied to this thread, I didn't pay much attention to the details of the PCM56; I assumed it was an I2S input device, but it is not.

This can be done, but it is not straightforward. You have the data and clock, and those should be OK. I'd check if both devices use the same edge: rising or falling; if not, invert the clock. You will need the Word Clock (WCLK or LRCLK) and a master clock. Your system may have a suitable master clock, or you may need to use a frequency multiplier chip (e.g. ICS511) on the bit clock to derive a master clock for the CS8406. Even then this will only work if your system's bit clock is continuous, not bursting. With some clever work, you can derive the WCLK from the LE signal and the enable signals for the sample/hold circuits.

You will need to make use of the oscilloscope you just bought to visualize the various signals, both the ones provided in your system and the ones you will need to create/derive. You can compare them to the timing diagrams in the data sheets to ensure that the timing and polarity of the signals looks OK.
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Old 1st April 2014, 04:31 PM   #18
Jidis is offline Jidis  United States
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Thanks for all the info guys. Even while not being able to do what I planned, I'd like to learn more about it.

Quote:
Originally Posted by abraxalito View Post
I'd be interested to know of any relatively modern kit using PCM56 or similar.
I should have specified there- None of this stuff would be considered modern by digital standards. The sampler and a couple others here are all from around 1990-91, and the things I ran across online were either similar (old music hardware) or fairly old CD players.

Quote:
Originally Posted by googlyone View Post
The input / implementation for the CS8406 is super simple provided you have an I2S interface to drive it. If you want one I can post it for you - the PCB is 2 by 3 inches.
Yes (if it's not too much trouble), I'd like to see it. After the d.out for the sampler idea was shot, I thought about adding a compatible ADC to the 8406 and sticking it in a mic pre or something here, but even the stuff I could find on that was way more elaborate than a pair of chips.

Quote:
As an aside, have you been able to dig into what other interfaces and data ports exist on the DSP in the system?
Unfortunately, no. I was surprised that I was able to get service docs at all for this thing, but they're not much more than some rough scans of the schematics. There are a couple notes scribbled on them about what frequencies to expect at different points, but nothing as specific as data formats,etc.

It is fun to look at and learn from. They've got a whole army of logic chips in there. I wasn't even aware that those multiplexer chips were used for stuff like that (reconstructing analog audio). I'll poke around in it some more to see what's there and what's hitting the PCM chip. When I looked at it before, I'm not sure I got valid results. The signal I got off the clock line was way up in the 8.x MHz range and the latch was repeating at a rate of 125kHz, IIRC.

Thanks again to all,

George
Attached Images
File Type: jpg Peavey DPM SP mainboard.jpg (798.3 KB, 81 views)
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Old 3rd April 2014, 10:03 AM   #19
googlyone is offline googlyone  Australia
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Adding S/PDIF output to PCM56P?
Hi,
I have attached a screen-dump of the CS8406, and also the PCB. If you want the ALTIUM files, just email me.

These interface to an ADAU1442 DSP (which has one tiny bug that does not set a bit in the SPDIF stream that I really wanted set!).

I really wanted to drive the DSP into the SPDIF input of my PC, and the damn PC was ignoring the data! The design below was "quick and dirty" - the layout is not super great but then again, it is a pretty simple circuit.

The CS8406 is driven by:
LRCK
SCLK
SDIN
MCLK (a multiple of SCLK - usually 64-256 Fs)

Looking at your application, you would have to be generating MCLK also. You could go a PLL, or as I suggested use a micro to receive the data and generate all these clocks for you. On a bit of thought, you would probably end up having to implement an ASRC in software to go with all the other bother you have too...


So onto your question of using an ADC and 8406 to make an SPDIF ADC...

How hard would it be to hook this up to a ADC and have an SPIDF output from the ADC? Dead simple. Relatively speaking - using the CS5381 ADC. Caveats below...

I have attached a schematic of the ADC that I use - which interfaces to the same DSP. In this application I have set the ADC to SLAVE mode, and it receives MCLK, LRCLK and SCLK from the DSP.

In MASTER mode, you simply need a 12.288MHz oscillator, and it generates LRCLK, SCLK and DOUT. If you connect these, and the MCLK, to the CS8406, hey presto you have a SPDIF ADC. No need for big clunky DSPs and stuff, not even one line of software. You can set it all by jumpers.

SPDIF Output.jpg

SPDIF Output_PCB.jpg

ADC Schematic.jpg

All these boards use copper one one side, tracks on the top and a few wire links here and there. I have done this as I tend to make the prototypes using "press and peel" film.
- I would not recommend doing this as pure single sided - it will work, but you might end up with degraded noise performance
- When I said "easy" above, the above design is not for someone who is learning the ropes. There is some moderately high speed digital stuff, and layout that will take a bit of care. That said, it does not require any supporting logic or microcontroller to set up - so you can get this going without much mucking around.

Some notes of caution:
- Some PC sound cards sample at a fixed rate and use an ASRC on the input. You can see the artefacts of this in the noise spectra as a smear of spurs (a long way down I do concede, but they are there)
- You might want to crank up the sample rate - I have not played with this, and am not sure at the sample rate PC interfaces will take. I am sure someone will know though.
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Old 3rd April 2014, 09:52 PM   #20
Jidis is offline Jidis  United States
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Appreciate it Googlyone!

I'll be looking over those pictures.

Did you ever get any idea of why the PC wouldn't see that stream? I've been following the whole "onboard SPDIF" thing since back in the days of the ISA Epoch sound card and have been impressed at how they've refined it and it's become so common these days. I've usually only used it on stuff here for output. I can imagine the SPDIF input/external sync thing might be a bit more messy.

George
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