Thanks, that's exactly the sort of thing I was hoping to find. It's
also interesting to note that the human ear is most sensitive to
midrange frequencies, say between 1000Hz and 4000Hz. So, even if you
filter out what's above 4KHz, you can make up for the lack of clarity
with a little amplitude. However, it's this filter that can make it
difficult to distinguish between an F and an S sound. The frequencies
most necessary to hear those sounds clearly are above 4KHz.
And no, amplitude is NOT a measure of how much air a snowboarder gets
above the half pipe rim! :-)
John
>>> "David L. Blair" 2/27/02 8:58:07 AM >>>
> 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=36661&t=36566
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