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  • Three PC, two slave system - quick question prior to more intensive investigation.

    Hi All.

    I have a three PC (all Win 7, 32 Gig RAM, SSD, etc etc) system. One is running Pro Tools 11 and the other two are slaves.

    Generally this system is all running quite well with VE PRO 5.

    However, I am really pushing the system hard with low buffer sizes (64 samples) on the Pro Tools 11 system. Partly because apart from the load of VI instruments I have, I am also using a load of hardware synths - so trying to mimimse latency on them. I know there are already some workarounds to this (e.g. print the external synths to audio after direct monitoring etc etc).

    However, I am playing with my set up to see how well I can get it to cope with a low audio buffer. And here is the basic question. 

    I have created a test here: https://soundcloud.com/mark-stuart-ramsay/further-test-latency 

    This is hard work for the three PCs (loads of samples and long sustains). Now I am getting some pops and clicks. They seem to come from the two slaves:

    1) They are random

    2) They do not appear at the same point upon each playback

    3) Looking at the meters on VE PRO server (both PC's), I can see visually where the drop / pop takes place (they dip)

    I can stop this happening by upping the buffer in Pro Tools 11.

    However, what I do not understand really is WHAT is the buffer actually doing in Pro Tools to STOP the pops (at higher buffer sizes). The drops clearly seem to be from the slaves ...... so ...

    ... does this give me / you a clue that I should be optimsing the slaves (e.g. disc, network adaptor etc)... rather than anything specific on the main Pro Tools PC?

    MArk


  • this might become a little bit tricky to find the in-depth origin of your issue ...

     

    basically the buffers on your PT system are responsible for latency depending on sample rate and resolution (bit-depth), since finally data has to be written and read to and from memory. the reason why we actually have a buffer is to provide a continuous audio stream, whereas data access (from the system to your storage) is alway chunked.

    roughly latency (in ms) is buffer size (in samples) / sample rate (in Hz) * 2 - double the sample-rate to cut latency in half, but the amount of data to be shifted within a given time also doubles.

    simplified speaking your buffer has to be filled with a new chunk of data after a certain amount of data has been *streamed away* - the smaller the buffer is, the more often chunks have to be requested (from disk or from network), both puts additional load on your system.

     

    devices involved in this process:

    - processor of PT computer and it's frontside bus

    - memory of your PT computer and it's chipset

    in case of VE PRO via network additionally:

    - network interface(s) on your PT computer

    - switch connecting to slave computers

    - network interface on slave computers

    - processor, memory, chipset and disk on your slave computers.

     

    similar to disk-access also network access works in chunks (depending on your network interfaces and switch between 1500 - 9600 Bytes per packet).

    since file-I/O has always a very high priority on your system, some other processes may have to *wait* for processor time until they can be executed.

     

    i'd start investigating on the slaves with a tool http://www.thesycon.de/deu/latency_check.shtml which has always been helpful to track issues followed by checking the network interfaces (do they process network I/O themselves or do they leave work to the OS) and the switch (is the backplane capable of transporting all incoming data).

     

    in a few cases it also turned up that other devices on the network are flooding the switch or a network interface with undisirable load (broadcast, multicast), so having the audio network seperated from others is always *good design*.

     

    good luck, cm

     

    edit: i forgot to mention that your audio interface (on the PT computer) is also important - in case it is USB (2.0) i would not recommend less than 128 samples buffer size.


    and remember: only a CRAY can run an endless loop in just three seconds.
  • Thanks CM for that really useful and detailed reply - really appreciated :)

    To be honest - I think I am just pushing the slaves too hard - or put it another way - the buffer on the main PC is just too low. 

    For example, I can play (via the slave) a whole load of VI synths (not sample based but use a lot of CPU) - like LUSH 101. I can run this all day into Pro Tools with 64 buffer (and no buffer set on the VE plugin) with no clicks or pops. However, load up Vienna Instruments, add the 8W library from 8DIO, some strings from Spitfire audio and things start to get 'poppy'. It all goes if I raise the main buffer to about 256 (or even 512). 

    Testing continues :)

    Mark