The Enhanced Voice Services (EVS) codec was standardized by the 3GPP in 2014 as the next generation audio codec for use over VoLTE. Industry watchers are calling this a breakthrough in voice communications. EVS is the first 3GPP conversational codec offering support for up to 20 kHz audio bandwidth that delivers high quality for conversational speech and for other audio applications such as music or mixed voice/music.
EVS is intended to replace AMR-WB as the 3GPP codec of choice. While AMR-WB supports speech bandwidths up to 7 kHz, EVS support goes right up to 20 kHz which is the limit of human hearing (young humans at least). AMR-WB was coined “HD Voice” when it replaced narrowband AMR. To distinguish EVS from AMR-WB, EVS is being referred to as “Full-HD Voice”. It’s sort of like how 1080 HD Video was referred to as “Full-HD Video” when it replaced 720p HD Video.
Figure 1 and Figure 2 compares the bandwidth of Full-HD Voice (fullband) with HD Voice (wideband) and PSTN / POTS (narrowband). Note that while EVS has support up to fullband, most vendors are choosing to initially deploy super wideband which supports speech bandwidths up to 14 kHz. However, EVS also supports wideband and narrowband and will automatically reduce the encoding bandwidth if not required by the input signal.
The requirements of the 3GPP while developing EVS were following3:
- Enhanced quality and coding efficiency for narrowband (NB) and wideband (WB) speech services, leading to improved user experience and system efficiency. This should also be achieved in interoperation with pre-Rel-12 systems and services employing WB voice.
- Enhanced quality by the introduction of super wideband (SWB) speech, leading to improved user experience.
- Enhanced quality for mixed content and music in conversational applications (for example, in-call music), leading to improved user experience for cases when selection of dedicated 3GPP audio codecs is not possible.
- Robustness to packet loss, delay, and jitter, leading to optimized behavior in IP application environments like MTSI within the EPS.
- Backward interoperability to the 3GPP AMR-WB codec by having some WB EVS modes supporting the AMR-WB codec format used throughout 3GPP conversational speech telephony service (including CS). The AMR-WB interoperable operation modes of the EVS codec may be either identical to those in the AMR-WB codec or different but bitstream interoperable with them.
The 3GPP managed to deliver on all of these requirements with EVS. Unlike any previous 3GPP codec, EVS supports a channel aware mode which adds partial redundancy to the encoded stream to improve performance under packet loss. EVS uses mode requests to communicate channel conditions at the receiver back to the sender, so that the sender can add more or less redundancy as needed. Another first for a 3GPP codec, is that EVS includes a jitter buffer which is compliant to the 3GPP TS26.114 jitter buffer requirements and uses techniques such as time compression/dilation to best handle incoming jitter. Being integrated with the codec, the EVS jitter buffer works seamlessly with the channel aware feature to provide channel conditions to the codec and to supply the appropriate redundant information in place of lost packets.
Radisys is the first in the industry to support EVS codec and associated capabilities in our Media Server MRF products. Our implementation has been carefully engineered and optimized to deliver Full HD audio quality across the full range of Media Server media processing functions, allowing service providers to offer superior audio quality experiences to their subscribers with minimized spectrum, while delivering efficient transcoding as required to maintain endpoint interoperability across VoLTE endpoints using AMR-WB, WebRTC, or other VoIP endpoints.
Please email email@example.com if you’d like to learn more about Radisys Media Server support for EVS codec.
1 Source: EDN Network Full-HD Voice: Understanding the AAC codecs behind a new era in communication, January, 22, 2013. http://www.edn.com/design/consumer/4405424/Full-HD-Voice--Understanding-the-AAC-codecs-behind-a-new-era-in-communication
2 Source: Ericsson White Paper: Evolved HD Voice For LTE, October, 2014. http://www.slideshare.net/Ericsson/evolved-hd-voice-for-lte
3 Source: 3GPP TSG SA Plenary Meeting #47, March 22-25, 2010, Vienna Austria. http://www.3gpp.org/ftp/tsg_sa/tsg_sa/TSGS_47/Docs/SP-100202.zip