Audio output quality

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

Answer

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.