TDM 400p and Noise on the line

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I wonder if anyone has any sugestions

I have had a TDM400 for a couple of years, and I have always had problems
with noise on the line, so tonight I have been doing some research and have
found that if I load the CPU dahdi_test has almost perfect results and no

Opened pseudo dahdi interface, measuring accuracy…
99.997% 99.999% 99.998% 99.997% 99.999% 99.998% 99.998% 99.998%
99.998% 99.998% 99.998% 99.997% 99.998% 99.997% 99.998% 99.998%
99.998% 99.998% 99.998% 99.997% 99.999% 99.998% 99.998% 99.997%

One thought on - TDM 400p and Noise on the line

  • I suspect that you might be seeing some effect of the CPU
    going into, and out of an IDLE state… possibly due to the
    use of the HLT instruction in the kernel’s idle loop, or
    possibly due to the ACPI BIOS (or the operating system
    “cpufreq” support code) changing the CPU clock rate or

    These sorts of changes in processor state might have several
    sorts of effects on the TDM400P card and the audio it is

    – Changes in processor state (e.g. core voltage or clock speed)
    can cause a brief interrupting in processing… the CPU
    instruction processing must sometimes be halted in order to
    allow the clock PLL to re-lock at a different rate and for
    the core voltage to stabilize. This might possibly be adding
    enough latency to interrupt service time to affect the card
    (e.g. losing some audio samples), or skewing the timing enough
    that OSLEC’s echo cancelling algorithms exhibit different

    – Changes in the amount of power being drawn by the CPU, when
    it goes from flat-out processing to idle, can be quite
    substantial in modern CPUs. Some of today’s multi-core CPUs
    dissipate on the order of 100 watts during full-speed processing
    but drop down far below that when idle. The amount of current
    being drawn by the processor will cause changes in the voltage
    on the motherboard’s +12 and +5 supply lines, and these voltage
    changes are likely to reach the TDM400P. If the TDM400P doesn’t
    have good on-board voltage regulation and noise filtering (and
    I suspect that it might not), then some of the voltage noise on
    the supply rails could leak into the audio. Similar problems
    exist with many PC on-the-motherboard audio interfaces.

    As to how to fix it? Well, you’re going to have to experiment.

    The first thing I’d suggest trying, would be to see if you can
    disable any sort of ACPI- or kernel-based “power saving” adjustment
    of the CPU’s clock speed and core voltage. This might involve
    disabling SpeedStep (or the equivalent) in the BIOS, or switching
    Linux from using the “ondemand” power governor to a single-speed
    one (either “performance” or “powersave” or “usermode”).

    Possibly, running Asterisk at a real-time priority might reduce
    the issue, if it’s timing-related.

    If it’s simply a matter of noise on the power rails, you may not
    be able to get rid of it at all easily… might have to change
    motherboards, power supplies, or switch to an external phone
    interface device which is inherently immune to electrical noise
    within the PC chassis.