Exercise 3.4: Different Voice Codecs

From LNTwww

Audiosignale von "Narrow–Band"
und "Wide–Band"

The development of the GSM standard after 1990 was accompanied by the standardization of various voice codecs:

  • With the first  "Full-Rate Codec"  $\rm (FR)$  from 1991 a reduction to the data rate  $13 \ \rm kbit/s$  was achieved, sufficiently low to be able to transmit a voice signal over a single traffic channel.
  • In 1994 the  "Half-Rate Codec"  $\rm (HR)$  with the bitrate  $5.6 \ \rm kbit/s$  was developed with the aim of being able to transmit two calls simultaneously in one traffic channel if required.  However, the quality does not quite reach the full-rate codec.
  • The  "Enhanced Full-Rate Codec"  $\rm (EFR)$  from 1995 represented a significant development based on the data reduction method  $\rm ACELP$  ("Algebraic Code Excited Linear Prediction"). The EFR codec delivers a data rate of  $12.2 \ \rm kbit/s$  and stands for the common quality standard in mobile communications nowadays.
  • In 1999, ETSI standardized the Adaptive Multi-Rate Codec"  $\rm (AMR)$  for GSM.  This provides eight different modes with data rates between  $4.75 \ \ \rm kbit/s$  and  $12.2 \ \ \rm kbit/s$ .  The AMR codec uses the ACELP method like the EFR codec.
  • The  "Wideband AMR"  $\rm(WB-AMR)$  is a further development of the original AMR.  It was standardized by the 3GPP consortium in 2001 and by ITU-T in 2002 and uses the frequency range from  $50 \ \rm Hz$  to  $7 \rm kHz$.  This corresponds to a "WideBand signal".



Notes:


Questionnaire

1

What tasks does a voice codec perform?

It is used for rate reduction of digitised speech.
It is necessary for error correction reasons.
It is used for coding and decoding.

2

What advantages does the  $\rm EFR$  codec offer compared to the  $\rm FR$  codec?

It is designed for broadband signals $($frequency range:   $200 \ \ \rm Hz$ – $7 \ \rm kHz)$ .
It provides better voice quality.
The improvement is due to the ACELP method.

3

What are the advantages of the  $\rm AMR$  codec over the  $\rm EFR$  codec?

It provides better voice quality.
It provides different modes and is therefore more flexible.
The lowest mode is identical to the EFR standard.

4

What are the characteristics of  $\text{WB-AMR}$?

The audio frequency range is  $50 \ \rm Hz$ – $7 \ \rm kHz$.
It provides modes between  $6.60 \ \ \rm kHz$  and  $23.85 \ \rm kHz$.
GSM is not sufficient for the higher rate modes.


Solution

(1)  Correct are the answers 1 and 3:

  • The required data rate is reduced by removing redundancy and irrelevance from the data signal.
  • The artificial word "codec" indicates that the same functional unit is used for both encoding and decoding.


(2)  Correct are the answers 2 and 3:

  • The EFR codec from 1995 is a significant development of the "Full–Rate Codecs" from 1991, whereby, among other things, speech quality is less impaired by background noise.
  • Like the AMR, the EFR codec is based on the data reduction method ACELP ("Algebraic Code Excited Linear Prediction").
  • The first proposed solution is wrong.  Like the FR and AMR codecs, the EFR codec is only designed for the telephone channel   $(300 \ \rm Hz$ – $3.4 \ \rm kHz)$.
  • For better intelligibility and to avoid a dull sound, there is also a mid-range boost and a low-frequency cut.


(3) Only the answers 2 is correct:

  • The advantage of the AMR codec over the EFR is its greater flexibility.
  • If the channel quality deteriorates significantly, it is possible to switch smoothly to a low-rate mode where transmission errors are less disturbing.
  • In addition, as with the "Half–Rate Codec", it is possible to have two conversations in one traffic channel.
  • The highest mode at  $\rm 12.2 \ kbit/s$  - and not the lowest - is identical to the EFR codec.  It is therefore obvious that AMR cannot provide better voice quality than EFR.


(4)  All answers are correct:

  • Nine modes are provided in wideband–AMR, but only five of them are used for mobile communications, namely those with data rates of  $6.60$,  $8.85$,  $12.65$,  $15.85$,  and  $\text{23.65 kbit/s}$.
  • The modes up to  $\text{12.65 kbit/s}$  have the advantage that a voice signal encoded in this way can be accommodated in a single GSM traffic channel.  For the higher rate modes, GSM/EDGE or UMTS is required.
  • The higher rate modes  $(15.85$  and  $\text{23.65 kbit/s})$  provide only a slight improvement for speech, but due to the larger frequency range, they provide a noticeable improvement for the transmission of music.
  • Both the wideband AMR and the higher modes of narrowband AMR show weaknesses here.  An even lower data rate gives extremely poor results with music signals.
  • The WB-AMR has a better voice quality than the NB-AMR with a comparable data rate  $\text{(12.65 kbit/s)}$.  Due to the greater bandwidth, speech sounds are more natural and sibilants such as "s", "f" and "sch" become more intelligible.