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When a digital device (PC, portable media player, etc.) plays a digital audio file (ogg, mp3, flac, etc.) the audio signal output is always the same, regardless of the device type/brand, am I right? Thus the sound quality should not differ. For instance, I have an iPod that plays an mp3 file. If I play the same file on a different brand media player the sound quality should be the same since it is the very same signal (no equalizers, or any sound alterations).

Is this true? If it is true then I reckon it is only the headphones/speakers that matter as far as the sound quality is concerned.

P.S: all of the answers are really nice! wish i could accept 'em all

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    $\begingroup$ In general, the speakers are the weakest link in an audio chain as far as affecting the quality of what you hear. Frequency response/resonance, distortion, etc. are far from ideal. $\endgroup$ – endolith Sep 19 '11 at 15:03
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    $\begingroup$ @sterz I've upvoted you so now you have more than 15 rep. So even though you can't accept more than one, please upvote the answers below that you like :) $\endgroup$ – Lorem Ipsum Sep 21 '11 at 22:46
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Unfortunately, the real-world gets in the way - there's a whole host of things between you and the "bit-perfect" waveform that comes from the MP3 decoder:

  1. As you noted, user controllable filtering (graphic eq for example) - this may be disabled.
  2. "improvements" to the sound made in the digital domain (for example compression to make it sound "louder" and bass boosting) - this may also be disabled
  3. The internal processing is often done at a greater bit depth than the output DAC capability - for example, 32 bit processing but only a 16 bit DAC. There must then be a process to reduce the bit depth. This may or may not include dithering of some sort. The results of this will sound different depending on the implementation.
  4. The digital to analogue converters may be of differing quality, which will impact on (amongst other things) the linearity and noise floor of the analogue output.
  5. The clock feeding the DAC will also have an impact on the performance - higher jitter will be noisier.
  6. The headphone driver will also have various analogue characteristics - eg. noise floor, frequency colouration etc.
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  • $\begingroup$ You're implying that the DAC only works in 16 bit. While this is certainly true for many (if not most) consumer devices, it's definitely not true for all devices. $\endgroup$ – leftaroundabout Sep 22 '11 at 0:47
  • $\begingroup$ True, I was aware of that when I wrote it, but thought it best to keep it simple unless anyone commented. I'll tweak the answer... $\endgroup$ – Martin Thompson Sep 22 '11 at 8:10
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For the output quality to be the same, not only would the final digital signal have to be the same, but the path from the digital data to your ear (D/A converter including any digital preprocessing, amp, induced noise into the analog wiring, transducers/speakers, spatial paths to your ear, etc.) would have to add no audible differences. This may or may not be true. Not all D/A converters are equally linear. Not all amp power supplies are equally shielded (especially really cheap ones). Etc.

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  • $\begingroup$ +1; Playing the same data (the same audio file) gives you the same sound quality on the same hardware. For example there are differences between an on-board sound-card and a non-on-board one. (I speak from experience here) $\endgroup$ – Iulian Şerbănoiu Sep 19 '11 at 5:56
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WAV or FLAC are lossless, so the digital data should be identical when fed to the DAC. Lossy formats like MP3 and OGG don't store a waveform, though. It needs to be reconstructed from sparser data, and it's possible for the decoder to differ in its interpretation of the data. However, for MP3:

Decoding, on the other hand, is carefully defined in the standard. Most decoders are "bitstream compliant", which means that the decompressed output – that they produce from a given MP3 file – will be the same, within a specified degree of rounding tolerance, as the output specified mathematically in the ISO/IEC high standard document (ISO/IEC 11172-3). Therefore, comparison of decoders is usually based on how computationally efficient they are (i.e., how much memory or CPU time they use in the decoding process).

So assuming the EQ and other enhancements are turned off, as you said, there should be no difference from file to DAC. The only differences would be after the DAC, in the quality of the analog circuitry, amplifiers, and speakers (which are usually the biggest cause of differences anyway).

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    $\begingroup$ +1 for the final comment about speakers - speakers (and headphones) are nearly always the "weakest link" $\endgroup$ – Paul R Sep 19 '11 at 21:40
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It is very unlikely that the audio signal output is always the same. First off, the audio HW (chipset) is different between devices but before the digital audio actually gets D/A converted there is a good chance that it has been heavily processed. The reason being that the processing compensates for poor quality loudspeakers and a far from optimal stereo setup. Even though you disable everything you can in menus on the device (phone or laptop) it is almost certain that the audio is getting processed in one way or another. On a laptop I once experienced very weird audio quality until I found that default some noise reduction and echo cancellation stuff was enabled. If you use a different media player it could also make a difference. The answer to your question is that sound quality is almost certain going to be different from one device type to device type.

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