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 | + | [[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 | + | *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 | + | *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 | + | *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 | + | *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 | + | *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 "'''W'''ide'''B'''and signal". |
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''Notes:'' | ''Notes:'' | ||
− | * | + | *The task belongs to the chapter [[Mobile_Communications/Similarities_Between_GSM_and_UMTS|Similarities between GSM and UMTS]]. |
− | + | *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 | + | *We refer you to the interactive SWF applet [[Applets:Quality_of_different_voice_codecs_(Applet)|Qualität verschiedener Sprachcodecs ⇒ Quality of different voice codecs]] <br>(based on "Shock Wave Flash", German language). |
− | *We refer you to the interactive applet [[Applets: | ||
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<quiz display=simple> | <quiz display=simple> | ||
− | {What tasks does a | + | {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 $\rm EFR$ codec offer compared to the $\rm FR$ codec? |
|type="[]"} | |type="[]"} | ||
− | - It is designed for broadband signals $($frequency range: $200 \ \ \rm Hz$ | + | - It is designed for broadband signals $($frequency range: $200 \ \ \rm Hz$ – $7 \ \rm kHz)$ . |
− | + It provides better | + | + 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 $\rm AMR$ codec over the $\rm EFR$ 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? | + | {What are the characteristics of $\text{WB-AMR}$? |
|type="[]"} | |type="[]"} | ||
− | + The audio frequency range is $50 \ \rm Hz$ | + | + The audio frequency range is $50 \ \rm Hz$ – $7 \ \rm kHz$. |
− | + It provides modes between $6.60 \ \ \rm kHz$ and $23.85 \ \rm kHz$ | + | + It provides modes between $6.60 \ \ \rm kHz$ and $23.85 \ \rm kHz$. |
+ GSM is not sufficient for the higher rate modes. | + GSM is not sufficient for the higher rate modes. | ||
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</quiz> | </quiz> | ||
− | === | + | ===Solution=== |
{{ML-Kopf}} | {{ML-Kopf}} | ||
− | '''(1)''' Correct are the <u> | + | '''(1)''' Correct are the <u>answers 1 and 3</u>: |
− | *The required data rate is reduced by removing redundancy and irrelevance from the data signal | + | *The required data rate is reduced by removing redundancy and irrelevance from the data signal. |
− | *The artificial word | + | *The artificial word "codec" indicates that the same functional unit is used for both encoding and decoding. |
'''(2)''' Correct are the <u>answers 2 and 3</u>: | '''(2)''' Correct are the <u>answers 2 and 3</u>: | ||
− | *The EFR | + | *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 | + | *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 | + | *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. | *For better intelligibility and to avoid a dull sound, there is also a mid-range boost and a low-frequency cut. | ||
− | '''(3)''' Only the <u> | + | '''(3)''' Only the <u>answers 2</u> is correct: |
− | *The advantage of the AMR | + | *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 | + | *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 $\rm 12.2 \ kbit/s$ - and not the lowest - is identical to the EFR codec. It is therefore obvious that AMR cannot provide better voice quality than EFR. |
'''(4)''' <u>All answers are correct</u>: | '''(4)''' <u>All answers are correct</u>: | ||
− | *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 23.65 kbit/s. | + | *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 12.65 kbit/s have the advantage that a | + | *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 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 $(15.85$ 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 | + | *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 | + | *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. |
{{ML-Fuß}} | {{ML-Fuß}} | ||
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− | [[Category: | + | [[Category:Mobile Communications: Exercises|^3.2 Similarities between GSM and UMTS |
^]] | ^]] |
Latest revision as of 12:28, 23 January 2023
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:
- The task belongs to the chapter Similarities between GSM and UMTS.
- The graph shows the magnitude spectrum of an audio signal and defines the characteristics "narrowband" and "wideband".
- We refer you to the interactive SWF applet Qualität verschiedener Sprachcodecs ⇒ Quality of different voice codecs
(based on "Shock Wave Flash", German language).
Questionnaire
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.