digidesign 442 I/O as standalone DAC for S/PDIF transport

[transport.nut]

Hi,

I've been given a Digidesign 442 audio interface box - the Nubus Mac and controller card is long gone.

Rather than put this into landfill as toxic waste, I was wondering whether anyone has any experience of getting these units to 'free-run' as DAC boxes - taking S/PDIF from the RCA input jack and putting the analog audio out of the XLR connectors.

The S/PDIF decoder is the Crystal Semi CS8412

The DAC/filter pair is AD1860/NPC SM5813

I've found datasheets for the CS8412 and AD186x - no luck at NPC.co.jp for the digital filter, sadly.

I've tried feeding 2-channel 44kHz PCM from my CD player into the S/PDIF input to see if the box has any autonomous capability - but it stays stuck on AES/EBU 48kHz :-(

Opinions about the practicality of this project welcome !

[rfbrw]

http://www.npcamerica.com/Datasheets...cts/sm5813.pdf

[rfbrw]

It's been a long time since I last saw one, but doesn't it have a sync input? If it expects a 48K wordclock signal or freeruns at 48k without one, you may have to change the sync internally or by some other means. Or does it lock to video?

[transport.nut]

Originally posted by rfbrw
It's been a long time since I last saw one, but doesn't it have a sync input? If it expects a 48K wordclock signal or freeruns at 48k without one, you may have to change the sync internally or by some other means. Or does it lock to video?

It has 2 BNCs on the back - Slave Clock IN and OUT

There is no word clock input, BUT...

It came with another box - the Video Slave driver - that I assume is the gizmo that locks audio sampling to video frame rates - this appears also to take a master word clock input and produce a slave clock signal - presumably for the 442 IO when it is required to sample video and audio together.

I know little about S/PDIF, but doesn't it have its own embedded clock in the data stream ?

So you think that a 44kHz square wave at TTL or EIA-485 line levels will persuade the box to accept S/PDIF 44kHz ?

thanks for the URL for the dig filter, btw !

[rfbrw]

When an external reference is required the system can be configured to ignore the SPDIF derived clock. Wordclock at Fs rates and TTL levels is what you need but be aware some models also used Superclock at 256Fs. Simply put you need to build or buy a sync pulse generator ideally one that covers both options.

[transport.nut]

Originally posted by rfbrw
When an external reference is required the system can be configured to ignore the SPDIF derived clock.

I don't want it to ignore the SPDIF clock, though ! And presumably the box is configured to do this by ProTools sending data from the Nubus card (gone) to the SCSI connector on the back of the box ? I had decided that if the complexity of the project escalated to the point where it required some form of embedded controller hanging off the back of the box SCSI connector in order to boot it into a '44kHz pass-though' mode, it would take forever, and I would never finish it. The address mapping for the 442's devices is certainly Digidesign's IP.

Wordclock at Fs rates and TTL levels is what you need but be aware some models also used Superclock at 256Fs. Simply put you need to build or buy a sync pulse generator ideally one that covers both options.

11.2896 MHz ? !!!! - at least that frequency is available as a crystal module.

Reading the datasheets, I can now see a bigger problem, though...

The CD data from the transport will be arriving as 16-bit words, but the output AD1860 DAC is 18-bit and presumably that's what ProTools originally sent from the host Mac - isn't this a source of incompatibility in getting the box to operate as a standalone DAC - the interface would never have been designed to incorporate the logic to internally bit-extend 16-bit words to 18-bits for the DAC ?

[rfbrw]

If all you want to do is run it as a standalone dac with no external sync, i.e. CS8412 - SM5813 - AD1860, then thats a doddle. All you need is a little pcb surgery.The wordlength problem insofar as it is a problem will be addressed by the digital filter.

[transport.nut]

Originally posted by rfbrw
If all you want to do is run it as a standalone dac with no external sync, i.e. CS8412 - SM5813 - AD1860, then thats a doddle. All you need is a little pcb surgery.The wordlength problem insofar as it is a problem will be addressed by the digital filter.

have you done this, or has anyone to your knowledge done this here was essentially my first question :-)

It's exactly the 'pcb surgery' required that I was trying to get info about e.g. which control pins to hack on each chip and what values to set them to - as the CS8412 datasheet mentions the 8 data formats it can be configured to decode, but nowhere relates these to real transport formats like CD-audio. Is it msb first, &c. &c....

[rfbrw]

Originally posted by transport.nut



have you done this, or has anyone to your knowledge done this here was essentially my first question :-)

It's exactly the 'pcb surgery' required that I was trying to get info about e.g. which control pins to hack on each chip and what values to set them to - as the CS8412 datasheet mentions the 8 data formats it can be configured to decode, but nowhere relates these to real transport formats like CD-audio. Is it msb first, &c. &c....

Putting these things together is a bit like Lego or Meccano. With enough knowledge and an ability to decipher datasheets, you can interconnect anything.
The SM5813 expects 16bit LSB justified data valid on the rising edge of bitclock (SCLK on the CS8412, Pin 12 and BCKI on the SM5813, Pin2). In order to use the embedded SPDIF clock, bitclock and L/R clock (FSYNC on the CS8412, Pin11 and LRCI on the SM5813, Pin28) have to be configured as outputs. This equates to mode 5 of the CS8412. To configure this mode the mode pins have to set as follows,
Pin 17 M3 = 0
Pin 18 M2 = 1
Pin 24 M1 = 0
Pin 23 M0 = 1
The SM5813 also requires a master or system clock on Pin 6 XTI.
With an external reference, system clock at 192/256/384/512Fs, bitclock at 64Fs and L/R clock at Fs would be generated. In this case all the clocks will be generated by the CS8412. The CS8412 only does 256Fs (MCLK Pin19), so this should be connected to XTI on the SM5813. To configure the SM5813 to accept system clock at 256Fs, /CKDV, Pin 4, has to be connected to V+ and /CKSL, Pin3 has to be connected to GND. You may also have to check /RST Pin 14. This may be connected to the computer port to allow an external reset. It can be connected to ground.
On the output side of the SM5813, the wordlength has to set to 18bits to match the AD1860. Connect /OW20, Pin 16 to V+ and /OW18, Pin17 to ground but they should already be set as should most of the output side settings. The connections to the dacs should need no alteration.

[transport.nut]

Originally posted by rfbrw

Putting these things together is a bit like Lego or Meccano. With enough knowledge and an ability to decipher datasheets, you can interconnect anything. ...[]

slack-jawed with wonder ! many thanks for taking the time to post such a comprehensive and detailed reply !