sound file resampling

[Ken Bouchard]

Here's the situation:
Some open reel tapes were originally recorded at 3 3/4 i.p.s. NOW, the only
deck available, only plays at the higher speeds of 7.5 and 15 i.p.s.
Assuming one of those tapes is played at 7.5 into my sound card, could I
merely sample at TWICE the normal rate (88.2 KHZ) then reset the rate to
44.1 once it's in the pc to get it to sound normal? I know this works since
I use this technique to get an old 78 to turn at a lower speed on the
turntable when it's damaged.
The problem now is, let's say I don't have that ANALOG tape, but I have a CD
of that original tape played at the higher 7.5 Can I rip that file from the
cd, get it into my Sound Forge software and resample to get it to sound
normal or am I at a dead-end? I've got a gut feeling that I'll have to have
the ANALOG tape available so I can get the sound wave through my sound card.
(it's an Audiophile2496) Anyone? Am I making sense?

thanks, ken

--
1st Class Restoration
"Put your old music on CD"
www.dvbaudiorestoration.com

[jixelub]

You're making sense and I don't see why it would make any difference
how the higher speed file gets on to your HD. What does going through
your soundcard do but turn it into a digital format anyway---it would
be exactly the same file on the CD. Am I missing something?...

Dsmith

[Logan Shaw]

Ken Bouchard wrote:

> Here's the situation:
> Some open reel tapes were originally recorded at 3 3/4 i.p.s. NOW, the only
> deck available, only plays at the higher speeds of 7.5 and 15 i.p.s.
> Assuming one of those tapes is played at 7.5 into my sound card, could I
> merely sample at TWICE the normal rate (88.2 KHZ) then reset the rate to
> 44.1 once it's in the pc to get it to sound normal? I know this works since
> I use this technique to get an old 78 to turn at a lower speed on the
> turntable when it's damaged.
> The problem now is, let's say I don't have that ANALOG tape, but I have a CD
> of that original tape played at the higher 7.5 Can I rip that file from the
> cd, get it into my Sound Forge software and resample to get it to sound
> normal or am I at a dead-end?

You can't resample it to get it sound normal. You need to tell the
software that, while the WAV file (that you ripped from CD) denotes a
sampling rate of 44100 Hz, the actual sampling rate should be 22050 Hz.
Which is a different process from resampling. (Maybe you know that,
but I just thought I'd clarify since you used "reset the rate" in one
instance and "resample" in another when it appears you should've
meant the same thing in both instances.)

The most obvious negative side effect, though, is that your CD of
the original tape played at double speed is still only sampled at
44100 Hz. So when you change the sampling rate without changing
the sample data, you're going to lose an octave of bandwidth (err,
um, I mean you're going to lose the highest octave of the audible
spectrum). You will only get sound up to about 10000 Hz instead
of up to about 20000 Hz.

Losing the upper octave isn't a problem with with playing 78s "too
slowly" because you're collecting extra information by taking
twice as many (or whatever) samples as you would need to if you
played the 78 full speed. But when you play a tape double speed
and don't double the sampling rate, you're getting 44100 samples
for 7.5 inches of the tape when you should be getting 44100 for
3.75 inches, so you're throwing away high-frequency information.

Oh, of course, the other reason it's not a problem for 78s is it's
highly doubtful they contain any useful information above 10000 Hz
in the first place, whereas 3.75 in/s tape could.

- Logan

[[email protected]]

In rec.audio.tech Ken Bouchard > wrote:
>Here's the situation:
>Some open reel tapes were originally recorded at 3 3/4 i.p.s. NOW, the only
>deck available, only plays at the higher speeds of 7.5 and 15 i.p.s.
>Assuming one of those tapes is played at 7.5 into my sound card, could I
>merely sample at TWICE the normal rate (88.2 KHZ) then reset the rate to
>44.1 once it's in the pc to get it to sound normal? I know this works since
>I use this technique to get an old 78 to turn at a lower speed on the
>turntable when it's damaged.
>The problem now is, let's say I don't have that ANALOG tape, but I have a CD
>of that original tape played at the higher 7.5 Can I rip that file from the
>cd, get it into my Sound Forge software and resample to get it to sound
>normal or am I at a dead-end? I've got a gut feeling that I'll have to have
>the ANALOG tape available so I can get the sound wave through my sound card.
>(it's an Audiophile2496) Anyone? Am I making sense?

If the CD is sampled with tape at 7.5 ips with a samplefreqency at 88.2 kHz
but "saved" with a 44.1 kHz tag. It should be simple as relabel it as 44.1 kHz.
However if it's sampled with 44.1 kHz frequency range is chopped above 10 kHz..
Also consider that the tape deck itself might not be able to produce
frequencies above 20 kHz correctly.

Considerations:
* Sample frequency of cd-record data.
* Frequency response of tape deck.

One possibility could be to connect the motordrive to a frequencytransformer.
Which controls AC motor speed by altering the frequency of the mains.

[Arny Krueger]

"Ken Bouchard" > wrote in message


> Some open reel tapes were originally recorded at 3 3/4 i.p.s. NOW,
> the only deck available, only plays at the higher speeds of 7.5 and
> 15 i.p.s. Assuming one of those tapes is played at 7.5 into my sound
> card, could I merely sample at TWICE the normal rate (88.2 KHZ) then
> reset the rate to 44.1 once it's in the pc to get it to sound normal?

It will sound more normal and it may be your best option, but there may be
some need for additional equalization touch up.

Playing a tape at twice the speed doesn't quite give you twice the bandpass.
There are high frequency losses that are not dependent on tape speed, such
as the effects of tape head inductance and other losses in the reproduce
head electronics.

> I know this works since I use this technique to get an old 78 to turn at a
> lower speed
> on the turntable when it's damaged.

So you haven't tried it yet?

> The problem now is, let's say I don't have that ANALOG tape, but I
> have a CD of that original tape played at the higher 7.5

Whoops! Bottom line - your transcriptions will hit a brick wall at about 11
KHz due to the characteristics of 44 KHz sampling. But, as I recall the old
3.75 ips tapes weren't good for much past that.

> Can I rip that file from the cd, get it into my Sound Forge software and
> resample to get it to sound normal or am I at a dead-end?

Seems like. I know I could make it work almost well enough with Adobe
Audition.

> I've got a
> gut feeling that I'll have to have the ANALOG tape available so I can
> get the sound wave through my sound card. (it's an Audiophile2496)

You're right on.

> Anyone? Am I making sense?

Yeah. Try it, and then do a little remastering. You'll probably get
something that will be useful, if not the ulitmate in hi fi.

[Scott Dorsey]

Dimitrios Tzortzakakis > wrote:
>Quote:"One possibility could be to connect the motordrive to a frequency
>transformer.
>Which controls AC motor speed by altering the frequency of the mains."
>most tape decks use dc motors.Correct me if I am wrong.These frequency
>transformers are very expensive, though,If it's a dc motor it's much easier
>to change its speed by the voltage.

No, most older open-reel decks use hysteresis synchronous motors that lock
to the line frequency. In general, though, operating these at half the
speed they are designed for is going to result in a lot of cogging and a
lot of flutter.

There are other machines that use three-phase motors with reference
oscillators, or DC motors with servo control. The Ampex ATR-100 is a good
example of a servo-controlled machine that can be run down fairly accurately
to the low speed of consumer decks without changing the capstan diameter.
The Revox A77 is a good example of a multipole motor arrangement driven
by a reference oscillator, which is also reasonable at low speeds.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."