What is HD Voice?

HD voice is a technology that delivers at least twice the sound as compared to a typical voice phone call delivered on a landline through the world's analog circuit-switched phone network.

"Plain old phone service" or POTS to be hip; Public switched telephone network or PSTN if you're more formal

Real world benefits from HD voice include:

• Better comprehension and clarity, especially in long detailed and/or technical discussions
• Clarity in understanding acronyms
• The ability to differentiate between and clearly identify others on a conference call
• Clarity and easier understanding in multi-national/multi-lingual conversation where you have non-native speakers and native speakers communicating in one or more languages
• More accurate transcriptions (both human and automated)

In short, everything involving voice is better in HD voice, be it simple person-to-person call , a 20 person international conference discussion, or a speech-to-text process.

The technology of HD voice

Sound is measured in hertz, or Hz. The human ear can typically hear everything between 20Hz and 20,000Hz. The higher the number, the higher (squeaker) the sound is until you move past 20,000Hz and into ultrasound frequencies only a dog can pick up.

A landline phone call captures and delivers sound in a range of 300Hz to 3400Hz, so there's a lot of sound information chopped off on both the low and high-ends of the scale. For simplicity's sake, a POTS call has a range of about 3.4 kHz (3400Hz). POTS calls are often called "narrowband" calls because they have such a restricted range as compared to what the human ear can actually hear and process.

Since an HD voice call is defined as delivering at least twice the sound range of a traditional phone call, an HD voice call will have a range of about 7 kHz -- or more. Wideband voice and HD voice are often used interchangeably since an HD voice call is "wider" -- more of a Hz range than a narrowband call.

In order to deliver twice or better sound than a POTS call, the first thing you need is a phone acoustically built to capture and deliver that extra information, so both the microphone(s) and a speaker/handset must capable of receiving and delivering across a 7 kHz or greater range.

Once sound is captured, it needs to be processed into digital form with a codec. The G.722 codec (more on codecs later) is generally considered the baseline for HD voice; it captures and delivers sound between 30Hz to 7000Hz.

Interestingly, a HD voice call using G.722 can be delivered on the same amount of bandwidth as its digital POTS equivalent of G.711 -- 64 kbp/s. If you are currently using G.711 in a VoIP phone system, you can switch to HD voice without needing more bandwidth.

Finally, two (or more, if it's a conference call) parties need to be able to talk to each other using the same voice encoding (codec) scheme. Within an organization/PBX domain, this is pretty easy -- turn on G.722, reboot the phones, and you're done. Communicating between different HD voice groups, or "islands" is more difficult because there's some Internet peering and interconnection issues involved, but service providers are working out the details to transparently provide HD voice calling.

What phones support HD voice?

A better question might be "What phones don't support HD voice?" All the major VoIP phone handset manufacturers -- Aastra, Allworx, AudioCodes, Avaya, Cisco, ShoreTel, Panasonic, Polycom, Siemens, and snom --support G.722 in their current (2010) phone lines going all the way down to the entry-level (i.e cheapest) model.