Taking serial audio outside the box with LVDS - diyAudio
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Old 23rd September 2002, 07:31 PM   #1
jwb is offline jwb  United States
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Default Taking serial audio outside the box with LVDS

So, S/PDIF sucks bad. RCA-terminated coax sucks really, really bad. I decided to use something better for my digital interconnect.

I started with El Cheapo, my JVC DVD/CD transport. Under the cover, there is an integrated stereo DAC with a 3-wire serial input. The data format is I2S. To move the I2S out of the box, I decided to use a quad LVDS transmitter (National DS90C031 in a SO-16) on the transport side and a quad LVDS reciever (National DS90C032 in the same package) on the DAC side. These parts are really small, but that's part of their advantage, and it could be worse: the 3.3V parts come in TSSOP. You can get them from Digi-Key which is a triple bonus in my book.

For the physical interface, I got some 3M surface-mount mini-d ribbon (MDR) connectors (part 10226-1210VE). Allied stocks these. This is a nice cable tech: 13 shielded twinax pairs in one cable, and the cables are widely available from computer stores. You could also use the more widely available 20-pin MDR connector, but Allied only stocks the through-hole version of that part. If you need to make your own cables, you could use twinax or twisted pair for runs up to a few meters.

After I got these parts and some tiny PCBs made up, everything else was cake. The LVDS parts need decoupling, and I went with the triple decoupling scheme described in National's data sheet. The LVDS tx/rx chips look pretty dinky next to a 1206 cap Also the reciever required a quartet of 100Ω terminators.

So far, no listening. The digital signal emerges at the other end of the receiver, which is good, but I need to modify the DAC to use the serial input instead of the AES receiver. But I'm pretty geeked about this interconnect, and I believe it will work better. It almost could not be worse than S/PDIF over coax.

There is a lot of room for variation here. SCSI VHDCI or HD80 connectors are suitable and widely available, albiet with 36 unused cable pairs. I would have used SCSI connectors if I had some laying about. Certainly you could also just use a group of twinax, or STP ethernet cable, or DVI.

Have fun, and to hell with S/PDIF.
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Old 23rd September 2002, 08:04 PM   #2
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LVDS transceivers are fairly used in the Mark Levinson D/A-converters. They carry the I²S-signals from the digital front end to the converter boards.
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Old 24th September 2002, 01:56 AM   #3
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Well that's a slight validation of this application, I suppose. Obviously LVDS has been used for many high speed applications, but I hadn't seen it in any audio equipment before.

Do you know of any audio (or video?) systems using LVDS to transmit serial digital audio between chassis?
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Old 24th September 2002, 07:47 AM   #4
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What about positive ECL (PECL) for this purpose ? Has anybody already tried this ? Are LVDS faster ?

I've attached the I2S (enhanced) input section of the Assemblage DAC3.1 It uses MC10H350 for receivers. Transmitters on the other side must be MC10H352, if memory serves.
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Old 24th September 2002, 12:05 PM   #5
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Default Alright!!!!!!

LVDS and ECL......I love it. Now we are getting somewhere.

To hell with SPDIF.

Jocko
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Old 24th September 2002, 04:45 PM   #6
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I dunno ftorres, your picture looks pretty complicated. PECL requires both-ends termination and uses more power than LVDS. I made the LVDS link with only 2 ICs and 4 passives.
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Old 24th September 2002, 05:11 PM   #7
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LVDS is used a lot for data transfer from one chassis to another in HDTV broadcast systems. 19.39 megbits/s Is that fast enough for ya? We use ECL and PCEL too. It just depends on what your doing.

Now just how did I know that?

Later
Bruce
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Old 24th September 2002, 06:13 PM   #8
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Not as complicated as it looks Just 'extra' components : input transformers, impedance matching resistors, pullup resistors, and so on. IMHO, the same goes for LVDS. If impedance matching is achieved (even with your ribbon cable - typically around 100R), you will have IMHO much better results than with no matching. Transformers are nice here, but you can try without... It's up to you. Whatever the technology behind, you must take care of the propagation between transmitters and receivers, and try to avoid any reflections caused by improper matching. Jocko ?
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Old 24th September 2002, 06:20 PM   #9
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With LVDS, you don't need the pullup resistors. That is a PECL-specific thing. Of course you terminate your cable with a 100Ω resistor. In fact LVDS doesn't work *at all* without the resistor at the end. But still, it is extremely simple: on tx->cable->resistor->rx. Pulse transformers not needed.
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Old 24th September 2002, 09:05 PM   #10
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Hi,
I'm new to this forum, and this seemed to be a good place to jump in since it's a subject near and dear to me.

I was planning on using the same approach with LVDS transmitters and receivers, although I haven't actually done it yet.

My thoughts had been to use a CAT5 flat cable (four twisted pairs) with an eight pin modular connector. It's reasonably cheap, easy to put together, and can easily be fitted on the transmitter circuit board in a small space with a square hole to plug in the cable on the back panel of most players.

One concern that I've had with this approach is how much jitter exists in the player to begin with. You don't want to simply transmit and capture the timing of a clock that's jittery to begin with.

My plan with the CAT5 cable is to use a precision oscillator *at the receiving end*, transmit it over an LVDS pair to the disk player to replace the player's original clock source, and then have it reflected back by a transmitter back to the receiver along with the data. The data would be latched into a buffer using the reflected clock, and read out of the buffer with the clock directly from the oscillator. This way both units are fully synchronous with each other, but with the clock source closest to where the data timing is most critical.

Anyway, that was my plan on how to approach this. It'll be at least a couple of months before I can actually turn boards to give it a try. I'd be interested to hear other peoples' thoughts and experiences regarding this topic.

Regards,
Brian.

P.S.
I think there's a lot to be said for using an ASRC on the receiving end. IMHO these really do a great job, even if S/PDIF is used.
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