HD Voice

This page is under revision to reflect the status of HD Voice and SILK technology in June 2011


HD Voice (High Definition Voice): Skype has always been associated with providing excellent voice quality. When Skype launched in 2003 it delivered “wideband” audio for Skype-to-Skype calls; in early 2009 Skype introduced its SILK technology which delivers superwideband  audio. Skype’s SILK technology is actually one of many “codecs” considered to deliver what is more often called HD Voice.

Compared to the legacy phone system’s narrowband audio, the crystal clear audio associated with HD Voice has been demonstrated to benefit users with:

  • Clearer overall sound quality
  • Easier to recognize voices, distinguish confusing sounds and understand accented speakers
  • Ease of deciphering words that have the close sounds of ‘s’ and ‘f’ and others, often indistinguishable over telephone lines.
  • Ability to hear faint talkers and to understand double-talk (when more than one person is speaking at the same time)
  • Reduced listening effort (decreased cognitive load), resulting in increased productivity and lessened listener fatigue
  • Better understanding in the face of other impairments, such as when talkers are using a speakerphone or in the presence of
  • background noise

In summary, Skype’s HD Voice results in a much clearer and crisper conversation in any Skype-to-Skype call, whether with a one-to-one call or as a participant in a Skype multi-party voice or video call hosted via the Skype client on a user’s PC. If you use a stereo headset you would say the other party is almost “in your head”. As one example, for customer service operation it means many fewer “Could you repeat that” interruptions and other misunderstandings caused by the narrower bandwidth of familiar telephone hardware. But the final proof is in the conversational experience.

Today Skype’s SILK technology is deployed not only in the PC Skype clients (Windows, Mac, Linux) but also in Skype for iPhone, Skype for iPad and Skype for Android.

Achieving superwideband audio involves not only the sound engine in the softphone (such as the Skype client) and the sound chip in PC hardware but also ensuring the microphone and speakers can handle that bandwidth.

Voice quality is reduced when making a SkypeOut call to a PSTN landline or, even worse, to a mobile device due to limitations imposed by the audio infrastructure design parameters of the PSTN and mobile networks. Ever wonder why mobile phones sound so squeaky? They are designed to handle around 2 KHz of audio bandwidth; the legacy telephone system only communicates 3.7 KHz audio.

Terms such as wideband audio have long been associated with describing Skype’s voice quality but, while Skype was a pioneer in providing such quality, others involved in telephony also are building hardware and softphones which provide at least wideband audio; as a result the generic term has become HD Voice.

HD Voice basically means an telephone system that communicates >8KHz of audio. While the human ear can theoretically hear from 80 Hz t0 14 KHz, most “talk” voice produced by our vocal chords is below 12KHz. While wideband audio supports up to 8KHz audio bandwidth, Skype’s superwideband SILK audio supports up to 12 KHz audio transmission.

For more see HD Voice: Priceless and Tom Evslin’s description and experience with a call from Vermont to Israel.


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