[DECtalk] DecTalk newbie

Wed Apr 20 00:22:30 EDT 2005

Some technical info to throw into the discussion, for those interested.

The Nyquist sample theorem essentially says that the sampling rate of 
a digital signal should be at least twice the highest frequency in 
the signal.  Therefore, a 11025 Hz signal should not contain 
frequencies above approx. 5500 Hz.  Most male and female talkers do 
not generate perceptible frequencies above 5000 Hz, so the 5500 Hz 
cutoff is not a big problem.

If you blindly up-sample a 11025 Hz signal to 22050 Hz or 44100 Hz 
without a low-pass filter, you are going to get some audio artifacts 
in the upper part of the audio spectrum.

In order for DECtalk to accurately generate speech at 22050 Hz or 
44100 Hz, it would have to calculate 6th, 7th, and 8th formants as 
part of the speech signal; currently it only calculates 5 formants. 
In nearly all formant-based text-to-speech synthesizers, formants 
above the 4th formant are usually held constant through the signal so 
adding 6th, 7th, and 8th formants would not really get you speech 
that is of a higher quality than what DECtalk gives us right now.

What I find interesting is that DECtalk is smart enough to tilt down 
the audio spectrum and only calculate 4 formants if you save the 
output at the 8000 Hz setting.

Reader note.  The above applies to the D.E.C. DECtalks; who knows how 
this applies to the Fonix DECtalk.  I don't say this tongue in cheek 
either--the Fonix DECtalk sounds to me as though it is of 
substantially lesser quality audio at 11025 Hz when compared with the 
D.E.C. DECtalk.

GUI Access

At 12:00 AM -0400 4/20/2005, Jayson Smith wrote:
>Hi,
>If you use a good audio editor, converting a sound file to a higher sample
>rate is not a problem, as these programs have anti-alias filters.  As far as
>the 11025 sample rate, that's probably the highest quality DECtalk
>generates, period.  I know that a file when played in the Speak43
>application sounds no different from the same file converted by the same
>application to a .wav file.  Think about it.  Back in the early 1980's when
>the DECtalk system was being designed, the technology of today just wasn't
>available.  As far as the general public of that time was concerned, when
>you bought music, you got it on some form of phonograph records or analog
>magnetic tape.  If you wanted to record something and store it in a form
>that could be easily played back or erased, you used some form of analog
>magnetic tape.  So if the concept of CD-quality sound even existed, it
>didn't exist in the minds of the general public as CD"s weren't available
>or, if they were, very few people had them.  So back then, a sample rate of
>11025 would have probably been perfectly reasonable.  Even now, telephone
>systems can't reproduce really high-frequency sound, and I'm sure back in
>those days the long-distance telephone networks were even less able to do
>so.
>Jayson.
>
>----- Original Message -----
>From: "Tony Baechler" <tony at baechler.net>
>To: "DECtalk Discussions" <dectalk at jaybird.no-ip.info>
>Sent: Tuesday, April 19, 2005 10:34 PM
>Subject: Re: [DECtalk] DecTalk newbie
>
>
>>  Hi.  I would like to expand on this a little, since I was messing around
>>  with Jayson's Daisy text file.  Actually, there are two serious problems
>>  with saving wave files from the demo.  First, 11 KHZ is the max sampling
>>  rate.  As most people know, that is fairly low quality nowadays.  I would
>>  say that 22 KHZ is a minimum for decent quality, but of course the only
>way
>>  to get that is by changing the sampling rate which causes problems as
>>  Jayson describes below.  The other problem is clipping.  That is actually
>>  what causes the distortion.  The best way to get around this is to save
>the
>>  sample at a lower volume, but I don't think the demo will let you do
>>  that.  The next best way is to record the speech with something like Sound
>>  Forge, RecAll, the Windows sound recorder or similar.  The best thing, of
>>  course, is to record directly from the hardware synth because clipping is
>  > minimal or nonexistant, but that is more complicated because you need a
>>  patch cord etc.  So no, there is no way to get it to harmonize with
>itself,
>>  but it isn't that hard if you have decent sound editing software.  Also I
>>  think there is some natural distortion that is part of the newer
>>  software.  My DEC Express uses the 4.2CD firmware and doesn't have
>>  distortion while the software Access 32 versions do, no matter
>>  what.  Actually I have noticed distortion with most software speech
>>  synthesizers.
>>
>>  At 07:32 PM 4/19/2005 -0400, you wrote:
>>  >As far as I know, there is absolutely no way to make DECtalk sing in
>harmony
>>  >with itself without using a wave editor.  DECtalk can only generate one
>>  >voice at any given instant in time.  There is an option in the DECtalk
>demo
>>  >to save a .wav file of the sound generated by the synthesizer.  Using
>that
>>  >and a sound editor, you could put several of those files together to make
>a
>>  >production, and that's what many people do.  Some, naturally, are of
>better
>>  >quality than others.  Some files have distortion because, presumably, the
>>  >voices weren't normalized before being combined, and/or the final product
>>  >wasn't normalized.  Some have aliasing.  Aliasing is a high-frequency
>>  >element added to a sound, where none was present before.  Aliasing is
>caused
>>  >by people converting the DECtalk wav files to a higher sampling rate
>using
>>  >certain poorly written tools for this purpose.  On the other hand, some
>are
>>  >of probably the best quality possible, given the limits of the DECtalk
>>  >system itself.
>>
>>
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>
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