[Asterisk-Users] Re: Problems with TDM400P card

[Mike Mueller]

On Tue, May 03, 2005 at 05:27:33PM +0000, Tony Mountifield wrote:
> In article <Chameleon.1115129364.adar0 at vegas>,
> Rich Adamson <radamson at routers.com> wrote:
> >  - a modified zttest.c run on both systems to show the delays in reading
> >    8192 bytes from the TDM card as 23,850 microseconds lateness on
> >    the old mobo, and 24,000 microsecond lateness on the new system. No
> >    significant change resulting from the differences in mobo, pci
> >    structure, interrupt structure, cpu speed, quantity of ram, kernel
> >    differences (v2.4 vs v2.6), etc.
> 
> See my response to your zttest-mod.c posting. I think it is 8000 bytes
> that are due every second, not 8192. That would make the timing on your
> new system pretty accurate if 8192 bytes are arriving in 1,024,000us.

Tony is correct.  You should expect 8000 octets/sec from a digital
sampler on a POTS line interface.

http://lists.digium.com/pipermail/asterisk-users/2005-May/105148.html

Reference:
http://www.ncta.com/industry_overview/cableGlossary.cfm?indOverviewID=41

Sample Rate - In analog to digital signal processing, the sample rate is
the interval at which samples of an analog signal are taken. The sample
rate for digital telephony, for example, is 8000 per second. 

http://www.freesoft.org/CIE/Topics/127.htm

G.711 is the international standard for encoding telephone audio on an
64 kbps channel. It is a pulse code modulation (PCM) scheme operating at
a 8 kHz sample rate, with 8 bits per sample. According to the Nyquist
theorem, which states that a signal must be sampled at twice its highest
frequency component, G.711 can encode frequencies between 0 and 4 kHz.
Telcos can select between two different varients of G.711: A-law and
mu-law. A-law is the standard for international circuits.

-- 
Mike