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I recently discovered an old cassette from 6 years before I was born, with my grandmother on my father's side playing piano and talking with my, at that time, 30-year-old dad. He says their voices sound a little fast.

I converted that cassette to an .mp3 file and opened Adobe Audition in order to change the pitch to try to restore their voices. But no matter how much I edit the audio properties with Realtime mode and putting the Stretch to 110%, 120%, or even 140%, it slows the same weird voice, but never achieve their true voice.

Is there a way to do that, or at least get close to that?

This is my first time with Adobe Audition.

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    Stretching the audio makes it longer /without/ changing the pitch, so that's not the correct operation. You want to slow the whole thing down, affecting pitch and time. Also, I would not use a realtime effect, but a rendered one. Look in Effects > Time and Pitch > Pitch Shifter. Jun 27 at 12:45
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    Oh, one more thing. Creating a lossy compressed file is the /last/ operation you want to do, after all of your processing and editing is done. Your working file should be a wav or aiff. Jun 27 at 13:39
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    Yeah, as Data said, first things first, for high accuracy and no generation loss you want to record it to a high quality WAVE format. If you want a compressed format like MP3, it should only be done as the final stage. Also, "normalize" is a technical term related to the amplitude of the signal. So I may edit those bits if I can think of a better term.
    – n00dles
    Jun 27 at 17:38
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    You guys are the best! I don't want to deviate from the technical subject, but I'm writing this comment super emotional hearing my mother in her late 20s and my father with 30-years-old. They have now 70 and 74 respectively and they don't sound exactly the same. Not to mention my dad didn't even remember that recording and hearing his late mother, my grandma, even playing the piano, got him in tears. Thanks guys!
    – Alain
    Jun 27 at 21:50
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    I forgot about that. You got it right in the answer. If the original was recorded with a battery powered cassette unit and the batteries were getting a little low, it would've recorded slow, resulting in a too-fast playback. OP should be good to go now. Jul 2 at 21:15

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So what you're attempting to do, is restore the pitch (i.e. "speed") of the recording. The reason it didn't work is that you used a timestretching algorithm instead, which leads to an unwanted outcome.

Some bits on timestretching...

Generally, the difference is, timestretching and pitchshifting uses an algorithm to effectively adjust only one parameter.
Timestretching adjusts the duration and not the pitch.
Pitchshifting adjusts the pitch (or frequency) and not the duration.
Different software may use different names for them, but, under the hood, they both use the same or similar algorithms, and none are perfect. You always get artefacts that the trained ear may notice, especially if it's a big adjustment or a real-time algorithm.

Pitch adjustment!

So what you needed was simple pitch adjustment. For tape, you could have just slowed the tape movement, this would have been an ideal solution, if possible. In the digital domain it's more simple but less technically ideal; you just slow the sample clock/decrease the sample rate, so that a given duration becomes longer and so the pitch lowers (Imagine the samples as evenly-spaced dots along the length of the tape if it makes it easier to imagine that slowing the sample clock = slowing the speed). But then you need to resample back to the original sample rate (So that it plays back at the adjusted speed) to finalise the process (In the analogy, we would add new dots at the originally-timed spacing, independent of tape speed). This is what the process you should have chosen does, "resample", and also, this is why it's important to use high quality audio files for processing(!!).

How to do it correctly in Adobe Audition...

First, the Pitch Shift process is for adjusting musical pitch, and it does not adjust the pitch and time together - it uses an algorithm to adjust only the pitch so don't use it. Instead, if you use
Effects > Time and Pitch > Stretch and Pitch,
you will see an option to RESAMPLE, and if you absorbed what I said in the last section, you should realise what this mean - It's the same as slowing or speeding up the tape, but in the digital domain. It removes the independent pitch/time algorithm. You should turn all other settings off and just use the semitone or percent slider.
So that a 50% reduction in length/12 Semitone raise in pitch on a 440 Hz pure tone looks like this:
(The processed wave is on the bottom)
image showing 880 Hz pure tone with Stretch and Pitch settings
As you can see, the 440 Hz tone gets resampled to 880 Hz and so the duration is halved. No other processing is done - just a simple resample.

A 200%/-12 semitone adjustment (as we're doubling the duration/halving the frequency) looks like this:
image showing 220 Hz pure tone with Stretch and Pitch settings

As you can see here, the Pitch Shift process (Effects > Time and Pitch > Pitch Shift) only adjusts the pitch. The duration remains the same:
image showing exactly the same duration after a +12 semitone adjustment

So DO NOT use Pitch Shift or any other independent stretching algorithm to restore the pitch on this occasion, as you want the TIME to adjust, too. So adjusting the speed through RESAMPLING is the way to go.

Note: The spaces in pitchshifting and timestretching are removed so that you know it's a specific process or effect, not a general description. As pitch shift could mean speeding up or slowing down the sound to specific intervals of frequency, i.e. musical pitch adjustment. Also, I didn't explain the pitch/frequency difference and, further, I should have used "frequency" on some occasions, but as this is a beginner, I didn't want to confuse things further, so I detracted from some technical terms.

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