Den 20101004 01:46:54 skrev Gale (Audacity Team) <gale@...>:
>
> "waffleman" wrote:
>> I am trying to generate a 4000 Hz tone with with a 8000 Hz project
>> rate. I
>> am simply using "Generate > > Tone..." with the following parameters:
>>
>>
>> Waveform: Sine
>> Frequency (Hz): 4000
>> Amplitude (01): 0.8
>> Duration 001,000 seconds
>>
>> After I click OK, a flat line gets generated. If I set the project rate
>> to
>> something higher, I see the
>> expected wave. According to NyquistShannon sampling theorm this should
>> be
>> no problem. Am I missing
>> something?
>>
>> My Audacity version is 1.3.12beta.
>
> 4000 Hz is just too close to half of 8000 Hz. Try generating 3990 Hz,
> then
> 3991 Hz, 3992 Hz and so on up to 4000 Hz. Every generation will be more
> degraded than the previous. Then listen to the difference if you generate
> those tones at a rate of 11025 Hz.
>
>
>
>
> Gale
>
>
>
I agree. It's simple math: fs = 8 kHz = 2·4 kHz = 2f ⇨ If first sample is
at 0°, then the next is at 180° and so on. sin 0° = 0, sin 180° = 0, sin
360° = 0 and so on.
If first sample is at 30°, for example, then every second sample will be
0,5 and the rest of the samples will be 0,5 (where 1,0 = maximum possible
level), since sin 30° = 0,5 and sin 210° = 0,5.
And if this doesn't make sense to some people, I guess I'd better mention
that I use comma for decimal separator. If that's not the default in your
country, imagine them as dots instead…

Kind regards
Johnny Rosenberg
