This came up in passing in another current thread but I didn't want to hijack that thread with this subject.
When the HD indicator on your IP phone is on:
- You ARE receiving audio encoded by a wideband codec
- You MAY BE receiving wideband audio
Explanation:
The bandwidth of the audio coming out of a wideband encoder is never greater than the bandwidth of the audio fed into the encoder. The encoder does not manufacture content -- it simply converts the content fed into it to a different format. Audio coming from the PSTN is never wideband audio: the PSTN simply does not support wideband audio. Thus when you are talking to someone who has landline service you are never receiving wideband audio. If your phone's HD indicator is on during such a call it is because the non-wideband audio from the PSTN was transcoded by a wideband encoder somewhere along the path to you.
If your ITSP and your IP phone or PBX both support the same wideband codec you can exchange wideband audio between yourself and your ITSP, but your ITSP has transcoded the non-wideband audio from the PSTN to the format of that wideband codec.
The only thing your IP phone knows is what codec was used to encode the audio it is receiving. It doesn't know whether or not that audio is wideband audio. (It could know that, but it would add to the phone's cost and the benefit wouldn't be worth the cost increase.)
In short, don't confuse the FORMAT of the audio you are receiving with the CONTENT -- they are two different things. Knowing what the FORMAT is doesn't tell you what the CONTENT is.
What is landline service?
Many people think that when they unplug their analog POTS phone from the jack in the wall and plug it into an ATA or a box from their ISP they still have landline service. I have seen this in a review on this site and heard this from friends. It isn't the type of phone you are using that determines whether you have landline service -- it's the service provider at the other end of the line. The only service provider that provides landline service is your local telephone company or a non-facility-based CLEC that is providing billing and customer service while using the facilities of your local phone company.
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