[AMRadio] AM Transmitter "quality"


Brett Gazdzinski Brett.Gazdzinski at verizon.net
Tue Sep 15 20:47:51 EDT 2009


Its all pcm (pulse Code Modulation).
One channel is 64 KHz, 8 bits, 8KHz sample, that is two samples at 4000 Hz.

The digital phone encoding has more resolution in the center of the 
amplitude range to help improve things (or lower bandwidth).

So the digital part of the network does pass to 4000 Hz, the backbone 
network has gotten great, gone are the analog (sideband) microwave systems, 
I can now call test numbers all over the US and get levels that are within 
.2 DB, and very low noise levels.

In most cases, the copper part has not gotten better, they don't want to pay 
to put air pressure on the cables anymore (keeps the water out), the craft 
level is in the pits, and a lot of cable is lead covered and OLD.

Cell phone companies used to compress the crap out of stuff using adpcm, 
Adaptive Differential Pulse Code Modulation and other ways to get twice as 
much traffic on one 64 Kb channel. With standard VOIP its really compressed, 
but in the demand aspect, they figure everyone is not going to be on the 
phone at once, and talking at the same time.
Special provisions have to be made to run fax or modems over VoIP, basically 
uncompressing it.

There are wide ranges of VOIP, VOIP is just a protocol, lots of customers 
are going with it for phone service, but with dedicated links into the CO, 
so in that case its just a way to get the calls to the customer building, 
the call is plain old pcm in the network.
that is a lot different from direct VOIP over dsl or cable modems.

Brett
N2DTS

----- Original Message ----- 
From: "Grant Youngman" <nq5t at tx.rr.com>
To: "Discussion of AM Radio in the Amateur Service" 
<amradio at mailman.qth.net>
Sent: Tuesday, September 15, 2009 3:25 PM
Subject: Re: [AMRadio] AM Transmitter "quality"


>
> On Sep 15, 2009, at 2:05 PM, Charles Ring wrote:
>>
>> While modern telephones can produce better audio (no carbon mics),
>> modern phone exchanges don't. Th e old exchanges passed whatever audio
>> was fed into them at least for local calls, but digital switching
>> has a
>> brick wall cutoff. I
>
> It depends on whether your exchange is still using a TDM switch or
> softswitch, and whether you're on a standard landline or VoIP
> service.  Big difference.
>
> Even TDM switches now go up past 3Khz -- the standard is in the 3.3Khz
> range, still far from adequate.  VoIP service is far superior with
> some services going up to the 7Khz range -- you can actually tell the
> caller is Granny, which isn't always the case on TDM infrastructure.
> On the landline here, my wife's sisters all sound the same -- I can't
> tell who's calling.  If they call on the VoIP line, that isn't the
> case.  There is discussion about improving cellular bandwidth, too,
> but with the typical lousy handset, it may not make a lot of
> difference.  Bad just sounds bad, not to mention the fact that all
> cellular has really done is kill our "old fashioned" expectations of
> what phone service should be.  We now fully accept glitchy, noisy and
> totally unreliable service at an absurd price as the norm .. :-)
>
> Grant/NQ5T
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