[Asterisk-Users] Pops in Call Recordings Tied to Dropped Audio in Calls

[Kevin P. Fleming]

Matt Roth wrote:

> That advice about not loading ztdummy came from our paid support, 
> including the bit about Asterisk falling back to the kernel for timing 
> if no other source is available.  It's concerning to me, to say the 
> least, that we are paying for misinformation.  We are now running 
> Asterisk in a production environment with no timing source.  We can't 
> change this until the end of the business day.  Luckily, only our MOH 
> *should* be affected, but we are trying to troubleshoot problems and 
> introducing any source of strange behavior makes that more difficult.

Please email your ticket number to me directly (_not_ to the list).

> We also wasted time last night testing whether or not removing ztdummy 
> solved our problem.  Paid support time (since we were on call with 
> Digium while doing the testing) as well as time that could've been used 
> to look into real solutions.  I'm not asking anyone to fix the problem 
> for us, but I would like some legitimate feedback on its possible 
> sources.  From the list, I consider it a gift.  From paid tech support, 
> I consider it a responsibility.

Agreed 100%.

> Does ztdummy fall to the kernel mode by default, or does it have to be 
> configured to do so?

On 2.6 kernels, ztdummy can use either the kernel ticks (jiffies) for 
timing, or the hardware realtime clock (RTC). By default it uses the RTC 
when built against kernel headers for 2.6.13 or newer; it can be 
manually configured to use the RTC for older kernels. The RTC is a more 
reliable timing source than kernel ticks, because they can be delayed 
small amounts due to the kernel performing process switches and the 
like, although usually there is not a great deal of difference. Also, on 
recent 2.6 kernels the jiffy frequency is adjustable at configuration 
time and no longer defaults to 1000 per second; if RTC mode is not used, 
then the kernel frequency _must_ be 1000Hz for ztdummy to work correctly.

On 2.4 kernels the situation is very different: ztdummy on 2.4 kernels 
uses a USB device for timing generation.