Difference between revisions of "Aufgaben:Exercise 3.4: Different Voice Codecs"

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[[File:P_ID2221__Mob_A_3_4.png|right|frame|Audiosignale von &bdquo;Narrow–Band&rdquo; <br>und &bdquo;Wide–Band&rdquo;]]
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[[File:P_ID2221__Mob_A_3_4.png|right|frame|Audiosignale von "Narrow–Band" <br>und "Wide–Band"]]
 
The development of the GSM standard after 1990 was accompanied by the standardization of various voice codecs:
 
The development of the GSM standard after 1990 was accompanied by the standardization of various voice codecs:
*With the first full-rate codec&nbsp; ('''FR''')&nbsp; from 1991 a reduction to the data rate&nbsp; $13 \ \rm kbit/s$ was achieved, sufficiently low to be able to transmit a voice signal over a single traffic channel.
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*With the first&nbsp; "Full Rate Codec"&nbsp; $\rm (FR)$&nbsp; from 1991 a reduction to the data rate&nbsp; $13 \ \rm kbit/s$&nbsp; was achieved, sufficiently low to be able to transmit a speech signal over a single traffic channel.
*In 1994 the ''Half-Rate Codec''&nbsp; ('''HR''')&nbsp; with the bitrate&nbsp; $5.6 \ \rm kbit/s$&nbsp; 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.
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*In 1994 the&nbsp; "Half Rate Codec"&nbsp; $\rm (HR)$&nbsp; with the bitrate&nbsp; $5.6 \ \rm kbit/s$&nbsp; was developed with the aim of being able to transmit two calls simultaneously in one traffic channel if required.&nbsp; However, the quality does not quite reach the full rate codec.
*The ''Enhanced Full-Rate Codec''&nbsp; ('''EFR'')&nbsp; from 1995 represented a significant development based on the data reduction method ACELP (''Algebraic Code Excited Linear Prediction''). The EFR codec delivers a data rate of&nbsp; $12.2 \ \rm kbit/s$&nbsp; and stands for the common quality standard in mobile communications nowadays.
+
*The&nbsp; "Enhanced Full Rate Codec"&nbsp; $\rm (EFR)$&nbsp; from 1995 represented a significant development based on the data reduction method&nbsp; $\rm  ACELP$&nbsp; ("Algebraic Code Excited Linear Prediction"). The EFR codec delivers a data rate of&nbsp; $12.2 \ \rm kbit/s$&nbsp; and stands for the common quality standard in mobile communications nowadays.
*In 1999, ETSI standardized the ''Adaptive Multi-Rate Codec''&nbsp; ('''AMR''')&nbsp; for GSM. This provides eight different modes with data rates between&nbsp; $4.75 \ \ \rm kbit/s$&nbsp; and&nbsp; $12.2 \ \ \rm kbit/s$&nbsp;. The AMR codec uses the ACELP method like the EFR codec.
+
*In 1999, ETSI standardized the&nbsp;Adaptive Multi Rate Codec"&nbsp; $\rm  (AMR)$&nbsp; for GSM.&nbsp; This provides eight different modes with data rates between&nbsp; $4.75 \ \ \rm kbit/s$&nbsp; and&nbsp; $12.2 \ \ \rm kbit/s$&nbsp;.&nbsp; The AMR codec uses the ACELP method like the EFR codec.
*The ''Wideband AMR'' &nbsp; ('''WB-AMR''')&nbsp; 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&nbsp; $50 \ \rm Hz$&nbsp; to&nbsp; $7 \rm kHz$. This is therefore based on a "WideBand signal".
+
*The&nbsp; "Wideband AMR"&nbsp; $\rm(WB-AMR)$&nbsp; is a further development of the original AMR.&nbsp; It was standardized by the 3GPP consortium in 2001 and by ITU-T in 2002 and uses the frequency range from&nbsp; $50 \ \rm Hz$&nbsp; to&nbsp; $7 \rm kHz$.&nbsp; This corresponds to a "'''W'''ide'''B'''and signal".
 
 
 
 
  
  
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''Notes:''  
 
''Notes:''  
  
*This exercise refers to&nbsp; [[Mobile_Communications/Gemeinsamkeiten_von_GSM_und_UMTS|Gemeinsamkeiten von GSM und
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*The task belongs to the chapter&nbsp; [[Mobile_Communications/Similarities_Between_GSM_and_UMTS|Similarities between GSM and UMTS]].  
UMTS]].  
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*The graph shows the magnitude spectrum of an audio signal and defines the characteristics "narrowband" and "wideband".  
*The graph shows the magnitude spectrum of an audio signal and defines the characteristics ''narrowband'' and ''wideband''.  
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*We refer you to the interactive SWF applet&nbsp; [[Applets:Quality_of_different_voice_codecs_(Applet)|Qualität verschiedener  Sprachcodecs &nbsp; &rArr; &nbsp; Quality of different voice codecs]] &nbsp;  <br>(based on "Shock Wave Flash",&nbsp; German language).  
*We refer you to the interactive applet&nbsp; [[Applets:Sprachcodecs|Qualität verschiedener  Sprachcodecs]].  
 
  
  
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<quiz display=simple>
 
<quiz display=simple>
  
{What tasks does a speech codec perform?
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{What tasks does a voice codec perform?
 
|type="[]"}
 
|type="[]"}
 
+ It is used for rate reduction of digitised speech.
 
+ It is used for rate reduction of digitised speech.
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+ It is used for coding and decoding.
 
+ It is used for coding and decoding.
  
{What advantages does the EFR codec offer compared to the FR codec?
+
{What advantages does the&nbsp; $\rm EFR$&nbsp; codec offer compared to the&nbsp; $\rm FR$&nbsp; codec?
 
|type="[]"}
 
|type="[]"}
- It is designed for broadband signals $($frequency range: &nbsp; $200 \ \ \rm Hz$&nbsp; bis&nbsp; $7 \ \rm kHz)$&nbsp;.
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- It is designed for broadband signals $($frequency range: &nbsp; $200 \ \ \rm Hz$&nbsp;&ndash; $7 \ \rm kHz)$&nbsp;.
+ It provides better sound quality.
+
+ It provides better voice quality.
 
+ The improvement is due to the ACELP method.
 
+ The improvement is due to the ACELP method.
  
{What are the advantages of the AMR codec over the EFR codec?
+
{What are the advantages of the&nbsp; $\rm AMR$&nbsp;  codec over the&nbsp; $\rm EFR$&nbsp; codec?
 
|type="[]"}
 
|type="[]"}
 
- It provides better voice quality.
 
- It provides better voice quality.
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- The lowest mode is identical to the EFR standard.
 
- The lowest mode is identical to the EFR standard.
  
{What are the characteristics of WB-AMR?
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{What are the characteristics of&nbsp; $\text{WB-AMR}$?
 
|type="[]"}
 
|type="[]"}
+ The audio frequency range is&nbsp; $50 \ \rm Hz$&nbsp; bis&nbsp; $7 \ \rm kHz$.
+
+ The audio frequency range is&nbsp; $50 \ \rm Hz$&nbsp;&ndash; $7 \ \rm kHz$.
+ It provides modes between&nbsp; $6.60 \ \ \rm kHz$&nbsp; and&nbsp; $23.85 \ \rm kHz$&nbsp;.
+
+ It provides modes between&nbsp; $6.60 \ \ \rm kHz$&nbsp; and&nbsp; $23.85 \ \rm kHz$.
 
+ GSM is not sufficient for the higher rate modes.
 
+ GSM is not sufficient for the higher rate modes.
  
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</quiz>
 
</quiz>
  
===Sample Solution===
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===Solution===
 
{{ML-Kopf}}
 
{{ML-Kopf}}
  
'''(1)'''&nbsp; Correct are the <u>solution suggestions 1 and 3</u>:  
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'''(1)'''&nbsp; Correct are the <u>answers 1 and 3</u>:  
*The required data rate is reduced by removing redundancy and irrelevance from the data signal  
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*The required data rate is reduced by removing redundancy and irrelevance from the data signal.
*The artificial word &bdquo;Codec&rdquo; indicates that the same functional unit is used for both encoding and decoding.
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*The artificial word "codec" indicates that the same functional unit is used for both encoding and decoding.
  
  
  
 
'''(2)'''&nbsp; Correct are the <u>answers 2 and 3</u>:  
 
'''(2)'''&nbsp; Correct are the <u>answers 2 and 3</u>:  
*The EFR&ndash;Codec from 1995 is a significant development of the ''Full&ndash;Rate Codecs'' from 1991, whereby, among other things, speech quality is less affected by background noise.  
+
*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&ndash;Codec is based on the data reduction method ACELP (''Algebraic Code Excited Linear Prediction'').
+
*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 Hz to 3.4 kHz).  
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*The first proposed solution is wrong.&nbsp; Like the FR and AMR codecs, the EFR codec is only designed for the telephone channel &nbsp; $(300 \ \rm Hz$&nbsp;&ndash; $3.4 \ \rm kHz)$.  
 
*For better intelligibility and to avoid a dull sound, there is also a mid-range boost and a low-frequency cut.
 
*For better intelligibility and to avoid a dull sound, there is also a mid-range boost and a low-frequency cut.
  
  
  
'''(3)'''&nbsp;Only the <u>solution 2</u> is correct:  
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'''(3)'''&nbsp;Only the <u>answers 2</u> is correct:  
*The advantage of the AMR&ndash;codec over the EFR is its greater flexibility.  
+
*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.  
 
*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&ndash Rate Codec, it is possible to have two conversations in one traffic channel.
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*In addition, as with the "Half Rate Codec", it is possible to have two conversations in one traffic channel.
*The highest mode at 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.
+
*The highest mode at&nbsp; $\rm 12.2 \ kbit/s$&nbsp; - and not the lowest - is identical to the EFR codec.&nbsp; It is therefore obvious that AMR cannot provide better voice quality than EFR.
  
  
  
 
'''(4)'''&nbsp;  <u>All answers are correct</u>:  
 
'''(4)'''&nbsp;  <u>All answers are correct</u>:  
*Nine modes are provided in wideband&ndash;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 23.65 kbit/s.  
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*Nine modes are provided in wideband&ndash;AMR, but only five of them are used for mobile communications, namely those with data rates of&nbsp; $6.60$,&nbsp; $8.85$,&nbsp; $12.65$,&nbsp; $15.85$,&nbsp; and&nbsp; $\text{23.65 kbit/s}$.  
*The modes up to 12.65 kbit/s have the advantage that a voice signal encoded in this way can be accommodated in a single GSM&ndash;traffic channel. For the higher rate modes, GSM/EDGE or UMTS is required.
+
*The modes up to&nbsp; $\text{12.65 kbit/s}$&nbsp; have the advantage that a speech signal encoded in this way can be accommodated in a single GSM traffic channel.&nbsp; For the higher rate modes, GSM/EDGE or UMTS is required.
*The higher rate modes (15.85 and 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.  
+
*The higher rate modes&nbsp; $(15.85$&nbsp; and&nbsp; $\text{23.65 kbit/s})$&nbsp; 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 WB-AMR 12.65 and the higher modes of (narrowband) AMR show weaknesses here. An even lower data rate gives extremely poor results with music signals.
+
*Both the wideband AMR and the higher modes of narrowband AMR show weaknesses here.&nbsp; 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 12.2 kbit/s even at a comparable data rate (12.65 kbit/s). Due to the greater bandwidth, speech sounds more natural and sibilants such as "s", "f" and "sch" become more intelligible.
+
*The WB-AMR has a better voice quality than the NB-AMR with a comparable data rate&nbsp; $\text{(12.65 kbit/s)}$.&nbsp; Due to the greater bandwidth, speech sounds are more natural and sibilants such as "s", "f" and "sch" become more intelligible.
  
 
{{ML-Fuß}}
 
{{ML-Fuß}}
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[[Category:Exercises for Mobile Communications|^3.2 Similarities between GSM and UMTS
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[[Category:Mobile Communications: Exercises|^3.2 Similarities between GSM and UMTS
 
^]]
 
^]]

Latest revision as of 12:28, 23 January 2023

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 speech 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 speech 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.