Much Ado About Everything

Today I was randomly reading Head-Fi, which I do extremely rarely (a few times a year at most) and I ran into some interesting new information. Well, it probably isn’t as much new to others as it’s to me, but this is my webblog after all, so news it is. Anyhow, it seems that a lot of people use or are advocating the use of RockBox firmware for iPods. What I find interesting is not that, but one of the alleged reasons for using RockBox – namely, hiss that can be heard when using iPod original firmware. The other reason – allegedly better sound quality – is another puzzle that is worth exploring.

I had iPod with me when I read it – it’s a 40G 4th generation one. I paused it, set the volume on maximum, listened carefully and then pressed my Sennheiser HD25-1 headphones against my ears… and then I heard it. Indeed it’s there. And it depends on the volume setting – if you lower it, it’s gone.

Let’s put it into perspective though. The hiss is very faint. Perhaps on very low impedance headphones it gets more evident, but it’s unlikely to be of any consequence in just about any listening situation. Furthermore, the question is whether it even matters when actually playing music. The fact that it’s only there when volume is at or near maximum points towards issues in digital domain. So the analog amplifier has low enough noise; perhaps iPod is playing digital zero signal (rather than real mute or DAC off) when paused, in which case one can hear the noise floor of 16-bit signal. I would say this is the most likely possibility. And if that’s true, it’s completely irrelevant to music listening – as long as you do get the full dynamic range that those 16 bits can provide.

So I did a RMAA measurement of my iPod (that’s headphone out, line out measurements are here). The results I got are very decent, though it looks like the noise is a bit higher than it should be and therefore it is not using the entire dynamic range. A 16-bit system is capable of about 98dB – for example my Flute 2 DAC measures like this in 16-bit mode (ignore measurements other than dynamic range and noise – this was before the DAC was further optimized). With iPod we got around 93dB – but I did have to increase input volume on the soundcard by 3dB (4dB for line out) to raise levels to what RMAA requires; this might mean that the noise floor and dynamic range are actually 96dB which would be perfectly in line with WM8971 DAC typical parameters (it is alleged that this one is closest to codec that my iPod is using). Another measurement on the web, of 3G iPod, is in line with my own regarding noise and dynamic range – about 93dB, though. These results are all superb for a portable device though

Note also that this measurement is done with no load on the iPod – it is only driving high impedance recording (line) input of my E-MU 1212m professional soundcard. When driving headphones, an ideal amplifier will still measure the same no matter the load; iPod and most other amplifiers will however change for the worse, and that change will be drastic. It’s not really their fault – there’s only so much you can do with low voltage, especially when you are also not allowed to use a lot of current. Some people – audiophiles, notably – would certainly consider sacrificing significant amount of battery life if it meant significant improvement in audio quality.

I do, however, also have an oscilloscope (or two). So I turned it on and measured the iPod, or rather just watched how the signal looks like on its output when it’s paused and at max volume. I got about 15mV peak to peak of what looked like broadband noise. At some point I thought I could isolate a 1MHz signal somewhere in there, but I could not reproduce it, so I will assume it’s mostly a broadband noise. If I capture a real-time “spectrum analyzer” (RMAA running in calibration mode, 24-bit!) screenshot, I get this. When iPod is turned off, I get this. There’s about 20dB difference between those two graphs across the board. If I play with the volume, with iPod still paused, I can easily see that the amount of this noise changes in sync with it. If you reduce volume to zero, you get below oscilloscope’s noise floor except for some high frequency spikes that seem to be around 150kHz.

I then turned on my IRiver iHP-120 which is incidentally running RockBox firmware. This player also produces hiss when paused – but the hiss is much less noticeable and does not seem dependant on the volume setting (however, when iPod’s volume is reduced, its hiss is lower than iRiver’s or even disappears). Therefore iRiver’s hiss is most likely due to analog amplifier’s noise. On oscilloscope, this player shows 10mV peak-to-peak, but there is a very strong presence of approximately 1.5MHz signal this time around, so a lot of the noise is way beyond audible range. I wonder why I was even able to hear any hiss – so I plugged headphones into my Flute 2. Even this amplifier seemed to create some hiss but at this point I was really wondering if it was audible or just my imagination; it cannot be measured by the oscilloscope’s as it is below its noise floor which is about 2mV peak to peak (at least with probes that I have which aren’t original so they pick up a bit of noise even when grounded). It could have also been the EMI from my room inducing noise in the long headphone cable – there’s plenty of EMI in this room full of cables and electronics.

In the end, I would say that iPod’s pause noise does not seem to present any kind of concern in terms of influencing the sound quality. Were it present with any volume setting, it could’ve been the reason of iPod not being able to reach limits of 16-bit sound (assuming my soundcard did not lose dynamic range for 16-bit due to having to increase its gain). However, that doesn’t seem to be the case.

I am not sure about the other claim – RockBox firmware somehow increasing audio quality. I will need to read up more about it – someone might have already found an explanation for it (I sure would hope so, this doesn’t look like very fresh news).

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