> John Neiberger wrote:
> What I'm trying to find out is why the original 4KHz limit on
> voice calls was put into place. It sounds like it was simply
> an arbitrary decision. 4KHz is sufficient for a telephone call
> and to provide clear calls that included higher frequencies
> might have added some technical complexities, perhaps.
>
> They also added a high-pass filter around 400Hz since most
> telephones can't reproduce low frequencies well and it also
> filters out some harmonics of 50-60Hz hum that might show up
> from time to time. That is concrete reason for including a
> high-pass filter and I wondered if there was a concrete
> technical reason for including the 4KHz low-pass filter. From
> the sounds of it there really isn't a technical issue, 4K is
> just a nice round number. :-)
I used three sources to answer John's query: "Voice over IP Fundamentals",
"Cisco Voice over Frame Relay, ATM, and IP", and Integrating Voice and Data
Networks". These are great books for anyone wanting to know more about
voice technologies.
Interesting Facts and Ideas I came across:
1) Human hearing is in the range of 200 Hz to 20,000 Hz
2) Human speech is in the range of 250 Hz to 10,000 Hz. Most of the
information comes from the middle frequencies. According to Nyquist, "Human
voice contains sounds that are more often Middle-pitched frequencies than
either High or Low pitched frequencies.
3) Frequencies greater than 4,000 Hz are filter out to limit crosstalk.
4) During the Analog to Digital conversion voice samples are put though a
process called Quantization. Quantization is the process of rounding
sampled values to the nearest predefined discreet value. Pulse Code
Modulation (PCM) is a Quantization process. PCM is also used to achieve 12
to 13 bits of voice information in 8 bit words. Two commonly used PCM's are:
mu-law (North America), and a-law (Europe). What you hear is not someone's
voice, but a representation of their voice.
5) Noise is a major issue when talking about voice quality. Noise is
constant problem for Analog signals. What is signal and what is Noise?
When a Analog signal is amplified so is the Noise, which in turn makes the
quality of Analog calls worst as the distance increases. Digital Calls are
less suitable to Noise than Analog calls.
6) Delay is a major issue when talking about conversation flow for two
reasons: 1) For a conversation to flow normally, the delay is receiving the
voice information must be less than 250ms. When the delay is more than
250ms, the human receiving the voice message will start to talk thinking the
human sending the voice message is at a breaking point in the conversation,
i.e.. both people are talking at the same time similar to a collision in
Ethernet. Delay is also important in how the voice packets are filled
during the Analog to Digital conversion. That is why ATM (ATM cell is 53
octets, 5 octets are header and 48 octets are payload) is a good method for
transporting voice packets because the delay to fill the payload section is
smaller than with other cell/packet types.
Answer: It does indeed seem that the 4,000 Hz mark was arbitrary in nature;
3,500 Hz or 5,000 Hz would work also. It is a "nice round" number to work
with. Simplies any math work. Middle frequencies carry the bulk of the
information and Human speech upper limit is 10,000 Hz amd 4,000 is near the
middle. The low filter is also to reduce the frequencies that carry less
information.
Hope this helps.
"Through Complexity there is Simplicity,
Through Simplicity there is Complexity"
David L. Blair - CCNP, CCNA, MCSE, CBE, A+, 3Wizard
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=36655&t=36566
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