Sound quality - Roon RAAT vs Roon Air

[David A]

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I have a few issues with Roon's explanations:
1. The Roon Ready dropout is not related to bit rate, it occurred with low bit rate internet Radio streaming as well as local & Tidal CD or HiRes files.
2. Changing to 100Mb/s switch won't change the raw PCM bit rate.
3. If it's a generic Devialet Ethernet issue, it should affect AIR as well. AIR and ROON Ready just different protocol over Ethernet, but AIR works perfectly over Ethernet.
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You're right, AIR and RAAT are different protocols and neither changes the PCM bit rate.

That doesn't mean they're the same. The PCM signal isn't what's streamed from Roon over ethernet. What's streamed is sequential fragments of the PSM signal bundled into packages with other things like extra code so the receiving device has the info required to reassemble the PCM stream in the right order, and code for an error correction system. In addition the receiving device, in our case our Devialet's, sends data back to Roon. That includes info required to tell Roon when to stream more data (the data is not streamed in real time as the music plays, it's streamed in bursts, the PCM stream is reassembled and fed into a buffer from where it's fed to the DAC at the correct speed for playback) and with RAAT the Devialet also sends back info about volume, tone control, and other settings like SAM which gets displayed by Roon in its signal path window but which aren't displayed for AIR because Devialet never built provision for that kind of info display into the AIR protocol.

One result of all of this is that RAAT uses more bandwidth to stream the same audio signal as AIR does. Back when the gigabit ethernet connection issue was under regular discussion here, someone posted test results showing how much bandwidth was used to stream the same audio signal via AIR and via RAAT. If I remember correctly the figures were something under 10 mbps for AIR and something a bit over 11 mbps for RAAT. Both streams utilise only a small percentage of the bandwidth available over a 100 mbps connection and a minuscule percentage of the bandwidth available over a 1000 mbps connection and neither should be a problem with either connection speed but the RAAT stream does present problems for the Devialet over a 1000 mbps connection while the AIR stream does not.
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It's a mistake to think of an ethernet stream as a simple stream of PCM audio like the stream sent over coaxial or optical digital connections. What is being streamed isn't an audio signal, it's a data signal which is contained in data packets which contain fragments of the PCM audio data plus other data required for data handling. AIR and RAAT are different protocols and they use different packaging arrangements. There's more non-PCM audio signal data contained in RAAT packets than in AIR packets because of the error correction code and other things which RAAT provides but AIR does not and that means that the RAAT data stream needs to be handled differently to the AIR data stream. Devialet hasn't yet got its handling of RAAT correct and it seems that what it has not yet got correct with RAAT isn't an issue for what needs to be done to handle an AIR data stream.

While the problem lies with Devialet's ethernet handling, it involves Devialet's handling of one data protocol, RAAT, but not a different protocol, AIR. Essentially it seems to be a protocol handling problem related to ethernet connections rather than an ethernet handling problem which affects all data sent via ethernet. Those 2 problems may sound similar but they aren't the same problem.