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How is ASVD done

Posted on 1998-04-23
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I would like to know what technique is used to do ASVD (Analog Simultaneous Voice and Data).  I know how DSVD is done (voice compression, the number of bits needed to send the coded voice comes off the total bits available for data.)  Can anyone describe how analog voice is sent during a data call?
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Question by:mjgaudette
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jlove1 earned 190 total points
ID: 1146667
First, I'll fully describe DSVD.. just to set the stage.
DSVD (Digital Simultanious Voice /Data) is a process by which analog voice is recorded, compressed, sent, decompressed, and played with modems, during which, the extra bandwidth is used by the modem for DATA transfers. DSVD is conducted TOTALLY at the HARDWARE level and is used for MODEM to MODEM connections only. I.E. The voice data CANNOT be routed over the internet under the current standards.

The ITU defined a standard known as v.70 that entails all the specifications for what has become known as DSVD. This specification was designed to work under 33.6 and 14.4 modems. 8kbps of the 33.6kbps (or 14.4kbps) is used for compressed voice data, except during times of silence, when the DATA uses the full bandwidth. The target for sound under the v.70 specs is VOICE, the compression schemes are used to be most effective with the human voice. The Voice delay should be about 120ms.

Usually, the modem is initialized in "DSVD" mode and will use DSVD whenever the phone is taken off the hook during data transfer. Most modems will RING the handset on the other side whenever one handset is off the hook during a data transfer. When the other party "answers", or, picks up, then the VOICE transfer begins.

ASVD on the other hand, is a standard whereby the modem will break to voice mode whenever needed.  The "S" in ASVD is really not accurate at all. It uses a switching technology that switches to to analog mode within 30 milliseconds of voice transmission. The sound is quite choppy for the most part, BUT music will sound MUCH better over ASVD. ASVD for the most part is FAR less expensive than DSVD, because of hardware required for DSVD.


Hope this clears the issue up for you.
Need anything more specific, just ask. I'm working on being the "modem" guru..

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by:mjgaudette
ID: 1146668
Just to be sure I understand:  With ASVD, the technique is actually to send regular analog voice whenever there is speech energy, and whenever there is not, the data packets are sent.  If one was looking at this from the modem's perspective it is a blank and burst scenerio?  Do you know how it is that the modems coordinate the switch from data to speech (without the carrier ending up blasting out the speakers) and then back again?  Do you know a good source for detailed information (book, web site, whatever?)  Do you know the standards designations for ASVD?
By the way, that was a great start at an answer.  I already understand the DSVD concepts very well.  And as a clarifying point, in DVSD the voice is not first recorded, it is compressed as it comes in, and the queueing of input speech should be kept to a minimum to ensure the natural progress of a conversation can be maintained.
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by:jlove1
ID: 1146669
After further research, I've come to a differing conclusion on ASVD. I found that the previous information that I've read is slightly untrue.

First, in my searchings, I've found that ASVD is a technology developed by AT&T. Secondly, it's not officially been given an ITU name as of yet, but will probablt emerge as V.34Q.. At the moment, AT&T refers to ASVD as V.ASVD or QADM.

In the proposed standards, ASVD not switching between voice and data as I previously stated, BUT MODULATING the Voice signal within the data signal. Never digitizing the signal, just modulating the signal and sending it over the data signal. In order to achieve this, the modem will have to drop it's bandwidth (i.e. stop using a "channel" or frequency to communicate on), so that there will be "room" within the 64K phone circut for the voice to travel down.

This suddenly makes more sense to me now that I've read this new (VERY HARD TO FIND) information. Some really interesting reading can be found at AT&T's website. The best of which is the following link:
http://www.att.com/att-technology/v10n1/pots.pdf

This PDF file isn't entirely on ASVD, but there are sections of the document that are VERY ENLIGHTENING. Hope this helps. Sorry for the incorrect information earlier-- what can I say, I was misinformed.

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by:mjgaudette
ID: 1146670
Thanks for the help.  I give an A for effort.
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