HD-Voice for cellular communication - Khariton, Dmitry, Vladimir, Oleg, Irina, Glory!

We enabled HD-Voice on the Beeline network in Moscow. Below are sound recordings before and after, as well as technical details.

The HD-voice mode is based on AMR-WB technology (Adaptive Multi Rate Rate Wadeband) - a broadband adaptive coding with variable speed, also this codec is known as the G.722.22 standard. In theory, a person can hear sounds in the range of 20 Hz - 20 kHz, but in practice it is enough to transmit sound in a much narrower range. When encoding speech in AMR-WB, a frequency band of 50Hz-7kHz is used, which is absolutely enough for full voice transmission. For example, the AMR codec uses a range of 300-3 kHz. The method can be used in 2G and 3G networks.
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Simply put, now the human voice will sound more natural , because it will cease to “squeeze” into a rather narrow band, the need of which was once dictated by the limitations of infrastructure. In addition, with HD coding, ambient noise has less of an effect on speech intelligibility.

How did we get to HD-Voice, and how does it work?

A person is able to hear a certain band of sound frequencies, usually it is assumed to be equal to 20Hz-20kHz. Transmitting the entire spectrum over wireless communication channels without compression is difficult, so the voice without cutting the band is transmitted only in wired systems. For speech compression, voice codecs or vocoders are used. Their side effect is the distortion of the transmitted voice by limiting the frequency band of the voice message.

The very first voice codec used in mobile networks was “Full Rate” - FR. Simultaneously with the FR, the Half Rate codec was introduced, the only purpose of which is to increase the number of simultaneously served subscribers, and, as its name implies, it occupies half of the FR codec band in the radio channel. However, the voice quality after transcoding to FR was not very high, and for some countries (for example, Arabic, where speech speed is high and many high-frequency sounds), it is completely unacceptable. Therefore, the FR codec has been reworked, and the “Enhanced Full Rate” appeared, providing significantly better voice quality, with a lower bitrate being created.

With the increasing penetration of mobile networks, the requirements for the quality of voice services began to grow, and a problem appeared with the deterioration of voice quality, with a low radio signal level from the base station. To solve this problem, a new codec was developed - “Adaptive Multi Rate”, which used the same frequency range for analysis and compression, but the algorithm was implemented in such a way that the codec bitrate dynamically changed, depending on the quality of the received signal. Due to this, an opportunity to provide excellent voice quality under good radio conditions, and for bad ones, to keep the conversation even where FR / EFR codecs could not work at all.

All 3G networks work only with the use of the AMR codec or its subsequent implementations, for example AMR-WB. But since the network remains a large number of subscribers using phones that support only work with EFR / HR codecs, the 2G network continues to work with all codecs (FR / EFR / HR / AMR FR / AMR HR) at once, providing voice to any phone.

Currently, voice quality requirements in mobile networks continue to grow, and the time has come to use AMR-WB technology. All the codecs described above use the frequency range up to 200Hz-3.4kHz for compression, while AMR-WB uses the voice message band at 50Hz-7kHz for coding. AMR-WB allows you to transmit twice the bandwidth, which gives an increase in the quality and saturation of the high and low frequencies of the transmitted voice.

Some characteristics of codecs are listed in the table:

Codec	Standard	Year of creation	Compressible frequency range	Generated bitrate
Full Rate - FR	GSM 06.10	1990	200-3400 Hz	13 kbit / s
Half Rate - HR	GSM 06.20	1990	200-3400 Hz	5.6 kbit / s
Enhanced Full Rate - EFR	GSM 06.60	1995	200-3400 Hz	12.2 kbit / s
Adaptive Multi Rate - AMR	3GPP TS 26.071	1999	200-3400 Hz	4.75 - 12.20 kbit / s
Adaptive Multi Rate - WideBand - AMR-WB	3GPP TS 26.190	2001	50-7000 Hz	6.60 - 23.85 kbit / s
Adaptive Multi Rate-WideBand + - AMR-WB +	3GPP TS 26.290	2004	50-7000 Hz	6 - 36 kbit / s (mono) 7 - 48 kbit / s (stereo)

Hint for curious programmers: a sample of the implementation of the AMR codec in the C language is available in the 3GPP TS 26.073 standard (you can easily find it, download it and try to implement it).

In the diagram below you can see a comparison of voice quality, according to the MOS (Mean Opinion Score) scale, depending on the codec used. The MOS scale is a subjective assessment of voice quality from 0 to 5, where 0 is a total lack of audibility, and 5 is live speech. G.711 codec is used in wired telephony, that is, this is the quality that you should hear from your landline phone (if it is, of course, not transferred to the IP channel, but this is a completely different topic).



Update: The comments rightly indicated that the average MOS score for the G.711 codec is about 4.1, this picture distorts reality a little.
As can be seen from the diagram (and taking into account the information from the Update), the AMR-WB codec provides, under good radio conditions, the voice quality is similar to the one we are used to when using a wired home phone.

For example, two fragments of a musical composition, which were compressed by AMR and AMR-WB codecs, you can compare the sound quality yourself and visually assess the difference in the sound picture created by different codecs.

AMR coded top track

Bottom track encoded in AMR-WB

Tests

We are not the first to launch this technology in Russia, so some developments have already been made. Our first part of the network with HD-Voice was built within the Moscow Ring Road, on the equipment of Ericsson. Activating AMR-WB took a long time: first, it was tested on one controller (with numerous drive tests to measure speech quality). And even after everything was tested on one controller, the subsequent activation proceeded in stages, controller by controller. The whole activation process took almost half a year.

Here are the sample samples from the final tests , which were evaluated by hardware using accurate measurements at different times.

Does it already work?

Yes, now the technology is implemented in the 3G network for Moscow. In order to use the mode, you do not need to make additional settings, it is activated itself, if both subscribers are in the 3G network and both their phones support the work of the codec.



The diagram shows the conditions that are necessary for the operation of AMR-WB. Now, as mentioned above, the codec is running on the 3G network, the next stage is to ensure the codec works between the 2G and 3G networks, then between the networks and between the operators.

What devices are supported?

Recognition problems

Human speech is such that in the radio and on a narrow band of the audio spectrum, the sounds of “C” and “F” are poorly distinguished. Couples “T” and “P”, “M” and “H” also suffer. Hence the mnemonic system with the names: “I dictate, number two-zero-zero, three Olga, as I understand it, reception?”. From here, things like the assignment of the most critical parts of the call sign to “p” grow up - for example, a sniper who in no case should confuse his call sign will most likely be “thirty-third” because it is very well discernible on air in narrow conditions. stripes.

Voice recognition is also growing with HD-Voice. This is important both for personal communication and for additional voice authentication gaining popularity, for example, in banks.