S/PDIF input to USB, is ITTS in I2S?

I am interested in a solution for S/PDIF input without SCMS detection to USB. The Cmedia CM6610A does support S/PDIF input to USB:

Supports S/PDIF input for 44.1K/48K/96K/192KHz@16/24bit
(96K/192KHz/24bit are available only in USB Audio Class 2.0/High-speed mode)

But sadly, I found no board utilizing that feature, I am thinking about designing a PCB for that chip or modding one existing board. But if I bother to design something, I could just build something completely new, that could support ITTS. Is the ITTS information kept when converting S/PDIF to I2S or would I have to build a custom S/PDIF decoder? Any insight where ITTS data is stored in the signal @Jac?

I found one relatively cheap commercial USB audio interface, that supports S/PDIF in, but it is according to the driver name based on the older Cmedia CM6206, which advertizes SCMS compliance and has lower resulotion. I will keep you up to date, when I receive the unit. B07DGR9M6M is the ASIN, avaible on European Amazon for about 15 €.

Those are a lot of questions :slight_smile:

The CM6610A seems like an interesting chip. I didn’t read the entire datasheet but it looks like there’s no mention anywhere of SCMS, so I wouldn’t be surprised if it will just decode the audio from SPDIF to USB and disregards the SCMS information completely. I can’t be sure though.

I think you may (also) want to look at the Teensy, particularly the Teensy 3.5 which I will be using for the DCC-i project, because it’s 5V tolerant. It can be programmed in the same way as an Arduino but it’s much more powerful. There is an Audio library available which makes it really easy to create audio projects. For example it should be relatively easy (a matter of drag-and-drop) to create a software project to read the audio from the I2S input, and let a Teensy stream it to a PC via USB. All you would need in theory) is an SPDIF receiver that outputs I2S to the I2S input on the Teensy. And you can use the other I2S port on the Teensy to stream the audio out again, e.g. to an SPDIF encoder. The outgoing audio wouldn’t have any SCMS restrictions; you’re the boss :slight_smile:

I think you mean: Is the ITTS information available in the I2S stream that comes out of an SPDIF receiver? The answer is no. The ITTS information is transmitted in the User Data subchannel of the SPDIF signal, and it’s not part of the audio, so it doesn’t end up in the I2S stream. My SPDIF receiver project has some information on how the subchannels are encoded.

As you may know, I did some reverse-engineering on the ITTS box of the DCC museum. The page of that project also has a link to the datasheet of the Mitsubishi M51581 SPDIF receiver chip (whic, by the way, was also used in some recorders). I don’t know if that chip is still available but I imagine that other SPDIF receivers work in a similar way: They convert the incoming audio to I2S and have separate serial output pins with only the bit stream for the User Data subchannel and the Channel Status subchannel. The ITTS box that the museum has, uses one microcontroller to decode the User Data channel and one microcontroller with a Teletext video generator to put the information on the screen.

It shouldn’t be too hard to design a circuit with an SPDIF receiver like the M51581, that has separate pins for the subchannel data, and a microcontroller that reads that data and sends it to a PC via serial or USB. Then you can write a program on the PC that implements the ISO/IEC 61866 standard for ITTS. As you may already know, it’s very expensive to buy a copy of the standard, and the standard is very complicated. There’s no doubt that the producers of DCC cassettes didn’t use the entire standard as it exists now. The Teletext generator in the ITTS box of the museum can only do European characters, for example. But even if you would want to write a program that only implements enough of the standard to work with DCC, it’s going to take a while.

===Jac

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Thank you for your great tips, I will take a look at it later. I bought a copy of IEC 61866 from a Russian seller for 1050 RUB, converted to 14.84 € and received it instantaneously. Long and complicated standard, but at least we have some documentation to work with. I opened your DAI dump and it seems to mostly contain a demo mode, take a look for yourself with a text editor. This means, that the undocumented part of S/PDIF -> ITTS should not be insanely complicated. I will try to write a decoder for the demo according to the standard.

I paid $270 for the standard but it should not be too hard to find it cheaper or free.

I’ve looked at the ROM dumps with a hex dump program too. It has some interesting strings including (if I remember correctly) the name of the software engineer who wrote it. Yes it has a demo mode which is activated by a switch on the back.

I don’t think I’ll be making a replica of the ITTS decoder of the DCC museum. It should be easy to reverse-engineer it further with a multimeter to trace some things out, but it’s not really necessary: I can guess pretty much all of the circuitry. With current technology, it just doesn’t make sense to make something like the ITTS box the same way. In the 21st century it should be much easier to accomplish the same thing with less hardware and more flexibility.

=== Jac

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I couldn’t find it for free but under USD 17 is good enough. The Mitsubishi M51581 seems to be present in all ITTS compatible 1st/2nd gen DCC recorders. The Philips TDA1315 in the 3rd gen players should support it as well, but it is stored in buffers to be accessed via registers, not streamed out. Same story as in many more modern S/PDIF receivers.

I fully agree that we shouldnt replicate it. But googling I found an interesting application note by ST Micro from 2018. The STM32F4/F7/H7 ARM microcontrollers are able to transmit receive S/PDIF including user data and have lots of IO options like USB, Ethernet, Micro SD, Multitouch Touchscreen and HDMI. A great value dev kit is avaible for under USD 90, what are your thoughts on it for a first look on the specifications? Model number: STM32F769I-DISCO

41 posts were split to a new topic: ITTS (DCC text) reimplementation