[For more on this topic, see Wideband VoIP: sounds good to me and Next-generation VoIP and the role of DSP.]
In Voice over IP (VoIP) applications, wideband voice has a very clear benefit to the end-user when using a device such as an IP phone—it simply sounds much better. In standard narrowband VoIP calls, the voice signal is sampled at 8,000 Hz, resulting in an effective voice pass-band of about 200 to 3,300 Hz. Wideband voice codecs offer double the sample rate, providing an effective pass-band of 50 to 7,000 Hz. What does this mean to the end user? It results in a much higher fidelity voice call, more akin to talking to someone in the same room rather than over a phone.
If the quality of a wideband voice call is so much noticeably better, some may question why all IP phones and VoIP devices do not simply switch to offering wideband voice? The answer lies in the trade-offs in both processing horsepower and the bandwidth required.
While there are multiple wideband codecs available, the most prevalent are G.722 and G.722.2. G.722 is a split-band version of the ADPCM waveform codec standardized by the International Telecommunications Union (ITU) in 1988. Waveform codecs like ADPCM do not consume many DSP cycles to process the voice, but they do require more bandwidth for the voice channel. In the case of G.722, it typically requires about the same digital signal processing (DSP) horsepower as the narrowband ADPCM G.726. However, the trade-off with wideband G.722 is that it requires a data rate of 48, 56, or 64 kbps due to the larger sample rate of the voice channel. In comparison, narrowband codecs like G.711 can operate at data rates of 32 kbps and lower.
G.722.2, introduced by the ITU in 2002, can help alleviate the extra bandwidth required by wideband voice, as it leverages the Wideband-Adaptive Multi-rate (WB-AMR) compression codec. G.722.2 is able to shrink the bit rate of the voice channel down to 12.65 kbps (down from 48 kbps in G.722) while offering improved quality. G.722.2 can be a great option for IP phone manufacturers seeking to offer high-quality wideband voice using a smaller bandwidth.
However, offering wideband voice with a compression codec like WB-AMR does come at a cost, as it increases the need for DSP processing horsepower. For example, G.722.2 requires four to five times more DSP cycles to process than G.722. Thus, it is easier to implement G.722.2 on a platform that includes a DSP, rather than on a platform that relies on a RISC processor for its processing power. In addition, the phone designer will probably make the trade-off of supporting G.722 vs. G.722.2, depending on what other features need to be in the IP phone, such as the number of conferencing channels.
Offering wideband voice is a great way to ensure the best possible quality of experience for users of VoIP equipment such as IP phones. IP phone and VoIP system designers may wish to choose from the following classes of codecs, depending on the most important system-design constraints:
- G.711, G.726: Offers narrow-band voice with low processing requirements, and with typical bit-rates of 32-64 kbps.
- Other narrow-band codecs with compression, such as G.729AB and G.723.1, offer bit-rates as low as 5.3 kbps, but require higher DSP processing horsepower.
- G.722: Offers wideband voice with low DSP processing requirement similar to standard G.726 ADPCM. However, it requires a bit-rate of 48-64 kbps per channel.
- G.722.2: Offers wideband voice and compresses the bandwidth down to 12.65 kbps per channel. The trade-off is that it requires more processing cycles than G.722.
About the author:
Brent Lorenz is the IP Phone Product Manager at Texas Instruments. Brent coordinates worldwide marketing and business development activities in the enterprise and residential telephony space. Lorenz has held a variety of positions at companies including IBM-Rational, Wind River, and VoIP software startups. Lorenz received a bachelors's degree in electrical engineering from Kansas State University and an MBA from the University of Maryland.