Have I understood spdif & word clock correctly?

  • First of all, I know there are tons of threads, both here and elsewhere, about connecting Kemper via spdif – I think I’ve read through them all by now… but just wanted to double check a few specific/practical things that I’m still not 100% clear on.


    Also, while I’m not new to music computer tech, or audio engineering, I’ve never used spdif, and never had cause to change word clock settings - so these are admittedly blind spots in my knowledge. Have been reading up on both, but bear with me if I’ve made any incorrect assumptions!


    1) Am I correct in understanding that to connect Kemper to audio interface (Presonus Quantum 2626) I just need a single cable, from Kemper OUT to audio-interface IN, and that this needs to be an RCA coaxial connector? I would only need a second cable (from audio interface OUT to Kemper IN) if I intend to re-amp?


    2) The word clock thing… I usually work at 48khz, and I see I can set the Kemper to this in it’s output settings. However it’s the master/slave thing I’m not totally clear on. Is it still the case that the Kemper has to be the master clock? If so, where do I need to set this? I can see options for word clock in my Quantum 2626 control software – it’s currently set to “internal”, so presumably I’d change it to "spdif" if Kemper connected. Is this all I need to do, or do I need to change any settings in my DAW (Cubase 11) as well?


    3) Will I need to change word clock settings each time I connect or disconnect the Kemper, and what happens if I forget? Or will my system automatically revert to using the Quantum 2626’s internal clock as master when I disconnect or power down the Kemper?


    4) And just to make sure I’m understanding the word clock correctly – by default this is controlled by my audio interface, right? And Cubase is slave to this? I used to assume my computer controlled the word clock - but having read up on it, it seems the computer itself has nothing to do with the word clock?


    Thanks!


    EDIT: - I've just noticed in the Kemper's output settings, above clock speed, you can toggle the button for "internal" or "auto" - so is "auto" for using the Kemper as slave to my audio interface's internal word clock? In which case I don't have to worry about switching back and forth any word clock settings when connecting/disconnecting the Kemper? Or is there any benefit to having the Kemper as master??

  • 1. Word clock is a specific way to sync digital audio devices (usually via a dedicated Word Clock output/input). The Profiler doesn't have this. What you mean is just 'clock settings'.


    2. If you set your interface to be slave to the Profiler clock, you can use 1 cable like you described. New Profilers made after mid 2019 (and all Kemper Stage units) can be slaved to the audio interface if you connect 2 cables.


    3. Synchronization (clocking) of digital audio happens in hardware digital audio devices, not on the computer, not in your DAW. Sample rate must match, so if you want to have your interface set to 48kHz, you need to switch the Profiler to 48kHz as well.


    4. Sorry, I can't tell you whether you interface automatically falls back to internal clocking when the Profiler is disconnected. You should try. :-) If it does, it might not automatically switch back to S/PDIF clock when you reconnect the Profiler though.

  • Many thanks for the info, much appreciated!


    I tracked down a suitable spdif cable thanks to one of your posts in another thread as well:


    https://www.thomann.de/gb/cord…CJsYW5ndWFnZSI6ImVuIn0%3D


    I guess my only other question is, is there any advantage in having the Kemper slave to the audio interface rather than the other way round? e.g. is keeping the audio interface as master likely to be more stable in some way, or does it really not make any difference which one is slave or master?

  • ...by the way, I found the same cable here with cheaper shipping, but it seems to have bulkier connectors - not sure if they would fit into the Kemper's recessed spdif ports?


    https://www.dv247.com/en_GB/GB…m-Rean/art-ACC0003469-000


    For some reason the same cable from Thomann (as linked previously) seems to have flat connectors, not the wider "Cinch" type - unless it's an old photo and they changed the design?? Cordial's product website shows the Cinch connectors too:


    https://www.cordial-cables.com/en/products/cpds-cc#

  • All of them should fit. Just one more advise:

    NEVER pull the cables off the Profiler. ALWAYS turn them slightly while you pull!

    These cables tend to sit very firmly on the socket and the sockets are SMD. So too much pulling force can damage the S/PDIF sockets.


    PS:

    I prefer to keep my master clock from the audio interface. I trust my interface more than external gear. And I don't want to switch back and forth all the time.

  • btw, the second cable, do I simply need to connect this from audio interface OUT to Kemper IN in order to enable the Kemper to be slave? Or do I need to route any sort of signal back into the Kemper through it?


    Also, I read today that unless you use sample rate of 44.1, and Kemper as master, SPDIF actually results in more latency than the main outs... which is not what I was hoping for... Will have to see if the additional latency is an issue, if it is, maybe I'll just have to work at 44.1khz for any Kemper intensive projects.


    One other thing - in Cubase's ASIO Driver setup page, there is a checkbox for "externally clocked" - does anyone know if I will also have to check this box (as well as changing audio interface clock to "SPDIF") if I use the Kemper as master clock?

  • Regarding the extra latency at sample rates other than 44.1 kHz, can you remember what the values creep up to? I remember there was a discussion on it but can't locate the thread now. I thought it was addressed in one of the beta releases but looking back through the change log I can only spot the 'reduced latency at 44.1kHz' note from v7.5. Is it possible this was the other way around, as in, the latency was high when operating at 44.1kHz?

    Under normal operating conditions (i.e. no S/PDIF) involved I measured latency at just over 2.5 ms to main outs, which is a good result considering the A/D D/A stages. When recording I monitor via an analogue desk, so direct from Kemper and mixed with the DAW playback, then shuffle the audio regions earlier in Pro Tools as required. If you have a means of reliably measuring your input latency, I guess you can apply this numerically rather than visually.


    As for which clock should be master, unless your interface lifts its output relays when re-clocking, I'd keep it on internal and avoid it blatting through your speakers/headphones. Some software also tries to control the audio interface settings (Pro Tools, for example), so loading up a session can change sample rate to match the previous settings. If the interface is the master, this can happen more elegantly without you needing to manually change the kemper, then re-clock the interface etc; the Kemper should just follow.

  • btw, the second cable, do I simply need to connect this from audio interface OUT to Kemper IN in order to enable the Kemper to be slave? Or do I need to route any sort of signal back into the Kemper through it?

    Correct, you only need to connect it for clock signal to be an available source to slave to - you don't need to route and pass audio.

  • Regarding the extra latency at sample rates other than 44.1 kHz, can you remember what the values creep up to? I remember there was a discussion on it but can't locate the thread now. I thought it was addressed in one of the beta releases but looking back through the change log I can only spot the 'reduced latency at 44.1kHz' note from v7.5. Is it possible this was the other way around, as in, the latency was high when operating at 44.1kHz?

    Under normal operating conditions (i.e. no S/PDIF) involved I measured latency at just over 2.5 ms to main outs, which is a good result considering the A/D D/A stages. When recording I monitor via an analogue desk, so direct from Kemper and mixed with the DAW playback, then shuffle the audio regions earlier in Pro Tools as required. If you have a means of reliably measuring your input latency, I guess you can apply this numerically rather than visually.


    As for which clock should be master, unless your interface lifts its output relays when re-clocking, I'd keep it on internal and avoid it blatting through your speakers/headphones. Some software also tries to control the audio interface settings (Pro Tools, for example), so loading up a session can change sample rate to match the previous settings. If the interface is the master, this can happen more elegantly without you needing to manually change the kemper, then re-clock the interface etc; the Kemper should just follow.

    I think Cubase also controls the sample rate based on project settings - e.g. if my interface is set at 48 and open a project at 44.1, it'll change it automatically. What I'm not sure about though is the "externally clocked" checkbox - I'm presuming I need to check this, as well as setting spdif in my audio interface, when using Kemper as master clock - I'll have to try it I suppose once my spdif cables arrive.


    btw, here's the thread about spdif latency: extra latency using SPDIF

  • Thanks for finding the thread - having looked through again, it was 44.1 kHz operation that produced the extra latency, but this was solved in subsequent releases as quoted.

    Yep, give it a go. It’s likely that the checkbox will stop the software from pulling up/down the sample rate on load. Go for whichever is the easiest workflow option.

  • I understood the thread slightly differently - isn't it that all clock rates used to produce higher latency than main outs, and that an update made it possible for reduced latency ONLY at 44.1khz and only with Kemper as master clock - but that at all other clock rates (or at any clock rate if Kemper was slave), the higher latency is still unavoidable for some technical reason (converters of some sort)?


    i.e. the only way to avoid higher latency with SPDIF is to work at 44.1khz with Kemper as master clock?

  • Just to report back on this now I've got my spdif cables:


    At 48khz, I measured the latency thru spdif to be only about 1.3ms more than thru main outs - so pretty insignificant really. And all worked fine with Kemper set to "Auto", hence didn't have to change any clock settings in audio interface or DAW.


    At 44.1kh - but only if with Kemper as master clock - the latency thru spdif was about 0.3ms less than thru main outs.


    So not much difference - will continue working at 48khz unless I ever really feel the need to shave off 1.6ms latency!


    Only odd thing was, at 44.1, with Kemper as slave I got a lot of noisy crackling/popping in the background - and yes, I double checked my audio interface and sequencer were at 44.1 too. Anyway, have contacted support re this.