GSM Voiice & Channell Codiing Seequueennccee
There are several schemes available for reducing the bandwidth required for a single voice
channel from the 64 kbps PCM requirement. When the GSM specification was being defined,
over 20 different voice coding schemes were initially considered. This number was rapidly
reduced to 6 schemes from 6 different countries before trials started.
channel from the 64 kbps PCM requirement. When the GSM specification was being defined,
over 20 different voice coding schemes were initially considered. This number was rapidly
reduced to 6 schemes from 6 different countries before trials started.
Speech Coding
• GSM transmits using digital modulation - speech must be
converted to binary digits
• Coder and decoder must work to the same standard
• Simplest coding scheme is Pulse Code Modulation (PCM)
• Sampling every 125 ms
• Requires data rate of 64 kbps
• This is too high for the bandwidth available on the radio
converted to binary digits
• Coder and decoder must work to the same standard
• Simplest coding scheme is Pulse Code Modulation (PCM)
• Sampling every 125 ms
• Requires data rate of 64 kbps
• This is too high for the bandwidth available on the radio
Each of the schemes involved various methods of manipulating the information contained
within speech to balance the best quality reproduction with the minimum bandwidth.
Speech obviously contains far more information than the simple text transcription of what is
being said. We can identify the person speaking, and be aware of much unspoken information
from the tone of voice and so on.
within speech to balance the best quality reproduction with the minimum bandwidth.
Speech obviously contains far more information than the simple text transcription of what is
being said. We can identify the person speaking, and be aware of much unspoken information
from the tone of voice and so on.
Advanced Speech Coding
• We cannot send the 64 kbps required by PCM• We need alternative speech encoding
techniques
• Estimates are that speech only contains 50 bits
per second of information
• Compare time to speak a word or sentence with
time to transmit corresponding text
• Attempts to encode speech more efficiently:
• speech consists of periodic waveforms -
so just send the frequency and amplitude
• model the vocal tract - phonemes, voiced
and unvoiced speech
• Vocoder - synthetic speech quality
Early vocoders which reduced the voice to just simple waveform information lacked the
human qualities which we need to hold a meaningful communication.
Hybrid encoders give greater emphasis to these qualities by using regular pulse excitation
which encodes the overall tone of the voice in great detail.
Eventually a hybrid of the German RPE-LPC1 scheme and the French MPE-LTP2 was chosen.
This became known as Regular Pulse Excitation with Long-Term Prediction (RPE-LTP).
human qualities which we need to hold a meaningful communication.
Hybrid encoders give greater emphasis to these qualities by using regular pulse excitation
which encodes the overall tone of the voice in great detail.
Eventually a hybrid of the German RPE-LPC1 scheme and the French MPE-LTP2 was chosen.
This became known as Regular Pulse Excitation with Long-Term Prediction (RPE-LTP).
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