Female opera singers probably hate it when people ask them to sing over the phone!?
OK, have we distracted you enough, John? ;-) Seriously, I think this was a great discussion. Thanks to everyone who contributed. Priscilla At 10:58 AM 2/27/02, David L. Blair wrote: > > 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 ________________________ Priscilla Oppenheimer http://www.priscilla.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=36715&t=36566 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
