[time-nuts] WWV/WWVH audio simulator?

Tom Minnis Tom_minnis at att.net
Mon Jan 6 23:53:17 EST 2014


I believe the Western Electric D1 channel bank was the first and the 
European standard came along later.  Then came the D2, the D3 and 
finally the D4 when integrated codecs finally came to be and it was 
practical to get rid of the common codec and do it channel by channel.  
I have tried to look at the spectrum of AM broadcast radio when they are 
taking phone calls and you can defiantly see the low frequency roll off 
starting around 300Hz for the guy on the phone and when the guy at the 
station talks, there are strong components below 100Hz.

On 1/6/2014 8:36 PM, Magnus Danielson wrote:
> Bob,
>
> It works the other way around. The standard Bell handset (103A I 
> believe the designation was) has the 300-3400 Hz response, and with 
> not so fancy analogue filtering, you can handle 4 kHz and thus 8 kHz 
> sampling rate. The ITU-T G.711 A-law (where naturally the americans 
> wanted their own, so u-law appeared) does non-linear pseudo-dynamic 
> compression into 8 bits. T1s cram 24 channels into one frame, and 
> adding 1 bit for framing, giving 24*8+1=193 bits per frame, giving the 
> 1544 kb/s rate. 193 being a prime have caused a bit of headache over 
> the years. In Europe, cramming 30 channels into a bundle was 
> preferred, and allowing 2 bytes for framing and signalling. In T1, you 
> do signalling by bit-stealing every 6th LSB on a channel. Caused some 
> grey hairs for modem designers back in the day, and followed along 
> over into the ISDN, as the primary rates was over E1 and T1. T1 also 
> had three different line-encodings, of which only one was really 
> transparent to all binary combinations.
>
> Oh the joy of early digital telephony. Many lessons where hard to learn.
> Synchronization was only one of them.
>
> Cheers,
> Magnus
>
> On 05/01/14 20:33, Robert LaJeunesse wrote:
>> The US POTS is digitized at 8KHz sample rate, so Nyquist says the 
>> highest frequency you can accurately digitize is 4KHz. Allow some for 
>> a (fancy digital) filter and 3400Hz is about the best you can expect. 
>> As for T1, almost right. The 8K samples per second are u-law 
>> processed to 8 bits each for transmission down the line, at 1.544 
>> Mb/s a T1 line handles 24 streams, plus 8K bits per second of 
>> supervisory data. Yes, a nice round 193 bits per frame.
>>
>> Bob LaJeunesse
>>
>>
>>
>>> ________________________________
>>> From: DaveH <info at blackmountainforge.com>
>>> To: 'Discussion of precise time and frequency measurement' 
>>> <time-nuts at febo.com>
>>> Sent: Sunday, January 5, 2014 1:53 PM
>>> Subject: Re: [time-nuts] WWV/WWVH audio simulator?
>>>
>>>
>>> This is by design
>>>
>>> The POTS (Plain Old Telephone System) specifies a bandwidth of 300Hz to
>>> 3,400Hz.
>>>
>>> http://en.wikipedia.org/wiki/Plain_old_telephone_service
>>>
>>> They are trying to cram as many channels into as little bandwidth as
>>> possible and the greater the frequency response they provide, the more
>>> bandwidth it takkes and the fewer channels they can provide.
>>>
>>> T1 lines were originally developed to bring 16 voice channels into a
>>> building that didn't have enough copper circuits.
>>>
>>> Dave
>>>
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