Aufgaben:Exercise 3.4: Different Voice Codecs

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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  (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  (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  ('EFR)  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  $12.2 \ \rm kbit/s$  and stands for the common quality standard in mobile communications nowadays.
• In 1999, ETSI standardized the Adaptive Multi-Rate Codec  (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   (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 is therefore based on a "WideBand signal".

Notes:

• This exercise refers to  Gemeinsamkeiten von GSM und UMTS.
• The graph shows the magnitude spectrum of an audio signal and defines the characteristics Narrowband and Wideband.
• We refer you to the interactive applet  Qualität verschiedener Sprachcodecs hin.

Questionnaire

1 What tasks does a speech 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 EFR codec offer compared to the FR codec?

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

3 What are the advantages of the AMR codec over the 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 WB-AMR?

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

Sample Solution

Solution

(1)  Correct are the solution suggestions 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 affected 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 Hz to 3.4 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 solution 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&ndash 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.

(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 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–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.
• 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.
• 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.