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I have lately been converting audio cassettes to digital sound, using Audacity. I am successful with many of them, but I encounter a peculiar problem during some of these conversions, that no web search has helped me to counter.

Specifically, when played from an Android device, the sound from the exported mp3 or ogg file, be it variable or constant bit rate (for mp3), is muffled. The same mp3 or ogg sound file sounds just fine when played from a computer. I have tried to play the file with vlc for android, and also an uploaded video version with the Youtube app, in Android 4.0 and 4.2, with phones of different brands.

Furthermore, the sound file that is distorted when played from the smartphone, sounds perfectly when heard through earphones, headphones,speakers - anything connected to the audio-out port of the phone. However, installing an app such as SoundAbout does not fix the problem, even after routing the sound through the speakers with the earphones plugged in. I have uploaded a sample Audacity project having the problem here:

https://www.dropbox.com/s/fsunz62wzkqe9zb/test.zip?dl=0

Any ideas?

atsag
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  • Just before this gets closed as off-topic.... you cannot possibly expect a phone to sound any good compared to almost any other audio device. If it sounds worse than other audio from 'proper' sources, that's probably because you recorded it too hot. To learn how not to do that is also off-topic for this SE site. – Tetsujin May 23 '16 at 19:10

2 Answers2

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While the discussion with Frank was constructive, the true reason behind this extremely distorted playback was not the poor frequency response of the smartphones, but rather, the problematic downmixing of the stereo recording, to mono. If, in Audacity, the given project is converted to mono (Tracks -> Stereo Track to Mono), the muffled sound heard from smartphone speakers can be reproduced on desktop speakers, laptop speakers - everywhere. According to the voted answer of this question,

https://dsp.stackexchange.com/questions/2484/converting-from-stereo-to-mono-by-averaging

in some old '70s and '80s cassettes a stereo effect was simulated with the L and R channels being in opposite phase - so when downmixing the two channels cancelled each other. This downmix necessarily occurs when a stereo file is reproduced in a smartphone that has a mono speaker system or when the music application produces mono sound. To counter this, we can either

i) apply a Phaser effect on the L/R channel in Audacity, with an LFO frequency of 0.001Hz and any LFO Start Phase or,

ii) invert one of the L/R channels,which is a better choice in the given case, due to the way the cassette was created.

Then, the audio result of the (tracks -> mix and render) command applied to the L and R channels will be as expected even when heard through a mono smartphone speaker system.

This can also be verified using an app like PowerAmp. Changing the audio balance of the stereo towards L or R, improves dramatically what is being heard.

Community
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atsag
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It is muffled because cassettes offer 40 dB dynamic range and if they are old, even less. This is fifty times poorer than CD. If it sounds so much different on a pc it is probably because you color the sound with some software. That be anything from EQ to compressor/decompressor or spatializer. You can use such filters before you convert to mp3 to embellish on the poor quality. A better use of old cassettes would be to use them as clay pigeons. Do you have a shotgun?

Frank Sixteen
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  • Well, then the only explanation I can think of is, on equipment you think it sounds good, the equipment doesn't further impair the quality. – Frank Sixteen May 23 '16 at 10:45
  • Thanks for your interest, but the digitized sound is perfect (compared to playback from a cassette player) even when played from an android phone - the only limitation is that it must be heard through earphones,headphones etc. No filters are used at all. Try it for yourself with the demo project attached. – atsag May 23 '16 at 10:49
  • Of course the sound isn't better after an analog to digital conversion. How it sounds on a cassette deck is irrelevant. If worse it only tells this deck is the weakest link. – Frank Sixteen May 23 '16 at 11:04
  • How can the speakers of an android phone impair the quality of an mp3 so noticeably? Try to export the project to an android phone to see what I talk about. – atsag May 23 '16 at 11:20
  • Because it sucks at low frequencies. – Frank Sixteen May 23 '16 at 11:24