Yesterday, Sprint and HTC announced the HTC Evo 4G LTE, the latest in the series of Evo phones from Sprint. While the device is largely unexceptional — it is simply a modified version of the HTC One XL that is heading to AT&T — there is a particular feature that stands out: HD voice. For the first time in several years, voice calling is getting an upgrade. But what is HD voice and what does it offer?

HD voice is an umbrella term which refers to technologies that enhance the quality of an audio call, or the audio portion of a video call. Depending on the calling system being used, HD voice may or may not be available.

Most people commonly experience HD voice when using VoIP platforms like Skype on their computers. However, most people do not consider Skype an HD voice platform because it is not a telephone system. With the transition to predominantly digital cellular/satellite communications, as opposed to the traditional terrestrial circuit-switched telephone network, HD voice became possible for telephony. Even better, governments around the world are now considering replacing traditional circuit-switched telephone networks (POTS) with optical fiber packet-switched VoIP networks.

For cellular (and possibly hybrid cellular/satellite) networks, HD voice has been in development since 2000. In the last two years, the specifications for HD voice on CDMA2000, GSM, HSPA+, and LTE have been finalized. However, this year is the year that device makers are finally being offered modem chipsets (from the likes of Qualcomm) that have full support for HD voice.

HD voice on CDMA2000 requires upgrading the 1X carrier to 1X Advanced. This new network infrastructure upgrade boosts capacity and range of a given cell by up to 70%, but more importantly it widens the 1X channel to about 300Kbps, giving more than enough room to support a brand new, higher quality audio codec. The EVRC-NW (Enhanced Variable Rate Codec – Narrowband/Wideband) codec for CDMA2000 systems is a redesigned codec that can scale from the narrowband channel sizes currently supported in CDMA2000 to new wideband channel sizes provided in CDMA2000 1X Advanced. Additionally, a multi-microphone noise canceling system is required to take full advantage of the codec. The new codec allows for properly encoding audio in the 50Hz-4KHz range at a 16KHz sampling rate, as opposed to half that for the older narrowband-only codec. Additionally, the wideband codec actually goes up to 7KHz instead of 4KHz, offering slightly better audio recording capabilities.

On the GSM side of things (which includes 2G GSM, HSPA+, and LTE), HD voice is largely a core network upgrade. The audio channel for the base stations needs to be widened a bit to make room for the AMR-WB (Adaptive Multi-Rate Wideband) codec, though. AMR-WB and EVRC-NW offer largely the same benefits and have the same requirements. However, the HD voice upgrade is easier on the GSM side, since no massive equipment upgrades are required at the base station/tower side.

There are restrictions for currently available HD voice implementations, though. It looks like the CDMA2000 and the GSM family variants of HD voice are not interoperable for the foreseeable future, so GSM calls to CDMA phones and vice versa will drop down to narrowband codecs that the current interoperability systems support. For CDMA2000 networks, every participant in a call must be connected to a cell site that supports CDMA2000 1X Advanced and each handset connected to the 1X Advanced cells must have a chipset supporting 1X Advanced and the new codec (otherwise it falls back to 1X and the older codec). For GSM networks, all handsets participating in the call must support the AMR-WB codec.

In LTE, HD voice is actually built into the VoLTE standard, because VoLTE uses AMR-WB. As long as the device supports AMR-WB, then HD voice is possible. The only thing left would be for the network operator to enable HD voice on the network.

For wireline networks (standard telephony), HD voice is not yet available. All currently available wireline telephony services (including VoIP based ones like Vonage) use narrowband speech codecs, so there’s no capability for HD voice. While SIP does support it (and is the basis for VoLTE HD voice support), most wireline networks cap the bandwidth to 12Kbps to prevent too much data from going through. Consequently, higher-quality speech codecs cannot be used.

Currently, there are no networks in the United States that offer HD voice. T-Mobile USA’s network is technically capable of it, since the new WiFi Calling feature uses the same technology as VoLTE. However, it is not currently enabled; it will be enabled when T-Mobile deploys LTE. Sprint has declared that it will enable it with the upgrade to CDMA2000 1X Advanced, but it will likely be mostly useless until late 2013. It will also support HD voice through VoLTE next year. No other CDMA carriers will offer HD voice through CDMA2000 1X Advanced. Verizon Wireless and AT&T will have support for the technology with VoLTE deployment, but only Verizon Wireless has announced plans to offer support for HD voice on LTE.

Sprint can trumpet the feature all it wants, but HD voice will remain a useless feature that hardly anyone will get to experience until late 2013 at the earliest. And even then, nearly all subscribers will experience HD voice through LTE, not CDMA2000 1X Advanced. If you want to experience HD voice, try using a computer-based VoIP solution for now.