MOST with analog sources

MOST
1

So I have a question about how analog sources are fed into a MOST system.

For example, in my Disco 3, I have AUX input which is analog. I assume that all data going into the audio amplifier in my system goes over the MOST system (will look through the documentation I have later today).

If that’s the case, does the head unit convert my analog AUX input to digital and then send it over MOST to the amp, which then converts it back to analog? Just curious how analog inputs work in this system when a MOST based amplifier is involved.

My assumption is that the AMP does not have analog input and is only provided data/audio feeds over the MOST optical bus. If I am wrong, then it probably just has an analog passthru over to the AMP.

2

Looking at some additional info in the MOST booklet makes it seem like I am correct in my assumption.

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3

hi @urieal

I have no experience at all with MOST, but I’ve read a lot lately - and I guess I have to read much more :sweat_smile:

I think you answered your question yourself already but I can try to explain it in my words - hopefully someone corrects me if I am wrong:

I guess it depends on the setting of your car where the analog AUX actually gets transferred to MOST. In my car (second picture in SAAB 9-3 II thread) you see, that the AUX is connected (analog) to the Entertainment Head Unit (EHU) which incorporates the radio and I believe it also creates the sounds for the direction indicator, warning messages and so on.
In my case the EHU therefore incorporates some kind of sound card and it is a node in the MOST ring as well. I guess every node in the MOST ring, which incorporates some kind of sound card, could translate the analog AUX signal to MOST.
If I am not wrong every node in the ring

  • receives all optical messages on the MOST
  • transfers them into electrical signals
  • can read all messages which match to its own address
  • can add new messages addressed to other node(s) → like e.g. the processed sound from AUX
  • sends its new and repeats all the messages that it received initially (unless they are corrupt) in the correct order by sending out a new optical signal for the next node

Hope it helps and does not confuse you!

4

Thanks for the reply. No confusion. I think you are right.

But the MOST bus, being a purely digital system, must transmit audio data using digital methods. Therefore, I assume the head unit in my LR3 (and the head unit in your SAAB 9-3 II) handles the analog to digital conversion (ADC).

It’s interesting, because you have a dedicated ADC chip. Since the MOST bus in my LR3 is 48kHz, I assume that the ADC sampling rate is also likely 48kHz in order to reduce the conversion steps. By contrast, CDs are sampled at 44.1kHz. So if the ADC is sampling at 48kHz (not something lower like 44.1), then it wouldn’t have to do a conversion internally to the 48kHz bus. It does have to do that for CD audio.

Of course, I could be all wrong about this. It may be able to send the digital PCM signal over the bus without conversion, I have to dig deeper to know for sure.

And, of course, there may be a straight analog signal pathway from the head unit over to amplifier that bypasses the MOST bus completely in my system. I will have to tear it down to be sure.

5

Sorry I missed this one! PCM does have to match the MOST speed since each timing interval in the bus expects a node to fill its routing table with the correct data.

You are correct in that aux (unless routed direct via analog to the amplifier) gets converted to digital, sent over MOST then converted back to analog. Hence the awful sound quality!