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A-link (PCM / I2S direct) specifications

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Readed this with interrest.

I strongly reduced my time into diy audio, but I really love one day to have a system with an active speaker driven from a multi DAC.

I do not know how it works but HDMI is more and more everywhere as is USB-C connectors too. Is it technically feasible with XMOS interface to have three I2S output for three DACs like the AYAs (or S5) and a single input USB/HDMI from the computer for active filtering ? I assume the digital streaming is multiplexed in time in the standalone cable from the computer. then each DAC will feed each channel, bass, medium, treble, to have an active filtered system. And not ADC, all from the computer with a fast industrial isolator chip to isolate the noise of the computer ground ?I2S migth be a problem to dispatch the three AYAs inputs, but LVDS protocole could be ok and more resilient for the TDA1541A which is ok with low voltage at its inputs chip made (can of worms here perhaps?)

What you think ? I mean conceptually speaking.

Regards, Eldam

Hello Eldam,

That is exactly what I meant with this connection. It includes four data lines, so in I2S it can carry up to eight channels (two channels multiplexed in each data line). In simultaneous data mode, each channel requires its line, so it is limited to four channels.

The previous year's USB interface board release (Mk3) supports either eight channels in I2S or four channels in sim. data mode.

Differential signaling would halve the channel count. You can consider if the LV (Low-Voltage) part of LVDS could be welcome or not for TDA1541(A), but going differential will also bring you again to jitter introduced by redundant (IMO) stages.


So four channels is ok for a 2 ways only if Sim mode is prefered.

Thanks Pedja.

Yes, sure. In such a scheme, I would be only cautious with tweeters, because simultaneous data mode can, because of its offset binary data format, introduce more pops and clicks than I2S (for instance, if you switch the input), and put directly connected tweeters at risk.

Quote from Pedja on 23 August 2023, 4:30 PM

It is two years since I published the A-link specs, and by now, plenty of Audial DACs with this interface have been shipped.

Some customers asked if these units may be compatible with other manufacturers' I2S sources, using HDMI connectors but nominally not complying with A-link.

The A-link pinout is indeed similar to the other I2S standards using an HDMI connector. The most important, when you look at it, are DATA (a.k.a. SD), BCK (a.k.a. SCK), and WS (a.k.a. LRCK), and they are at pins 1, 4, and 7, respectively. If your I2S source uses such a pinout, it is possible that it may work with A-link too. Please note, if your source pin 1 is Data (SD) negative, the signal polarity will be inverted (with very negligible DC offset as well), and if pin 7 is WS (LRCK) negative, the channels will be reversed (and there will be a delay between channels equal to 1/Fs, which will be also negligible, but only so long as you don't listen with headphones), but in terms of the fundamental operation, it will still work.

What still matters is the voltage.

By proposing the A-link, my main goal was quality. For this reason, I proposed a connection that uses "native" (CMOS/TTL) voltages. Practically speaking, the CMOS/TTL nowadays means mostly the signal swinging up to 3.3 or 5 Volts (triggering around half of this voltage for CMOS or around 1.3 V for TTL).

If your source uses LVDS, it is important to understand that the LVDS swing can be anything above 100 mV and is typically between 200 mV and 400 mV (differentially, loaded), although it can be also higher. Its typical offset voltage is 1.2 V, so what you may expect as a typical LVDS source is a signal swinging between 1 V and 1.4 V. LVDS, of course, also implies the negative poles, but the A-link DACs will simply ignore them.

Audial A-link specifies 2.3 V as a minimum for the high state. Audial DACs may work with a bit lower voltages, but such an operation is not guaranteed. So, if your source is nominally LVDS, you should contact the manufacturer and ask for information on the actual output voltage swing of their sources.

Unfortunately, I am not in a position to acquire different I2S sources to check if they will work with A-link or not, sorry. However, according to the feedback I had from the customers, I can report that the following units worked well with Audial S5 DAC A-link input.

XingCore AF200 USB Bridge ("Reverse the Phase" should not be checked) - credit to Bernd

Pi2AES 2.0 (W1 jumper 5-6 out) - credit to John

Everyone else who successfully tried Audial DACs with other manufacturers' sources with such an interface is welcome to report here.

I can confirm Mercury Streamer V2 (HDMI LVDS output) works with an Audial S5 (HDMI A-Link input) with the 4 dip switches in the off position

Thanks, Pablo.

I can also confirm that the USB-to-I2S gadget named Tenealay, based on the Amanero board, also worked well with the S5 A-link input. Its positive poles pinout is identical to the A-link pinout, and I checked its signal voltages too, and although they are nominally specified as LVDS, they swing between 0 and 2.4 Volts, so everything fits well with the A-link specs.

(I was not that happy with its jitter performance, though. I might have missed to find the best way to set its output interface, but apparently the major issue is that it is supplied from the USB voltage - in cases like this, the USB conditioners might be welcome.)

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