Modern Consumer Sound Appreciation

Over recent months, I have been racking my brain, trying to answer questions I have, about how sound that was compressed in the frequency-domain, may or may not be able to preserve phase-information. This does not mean that I, personally, can hear phase-information, nor that specific MP3 Files I have been listening to, would even be good examples of how well modern MP3s compress sound. I suspect that in order to stay in business, the developers of MP3 have in fact been improving their codec, so that when played back correctly, the quality of MP3s will stay in line with more-recent formats that exist, such as OGG Vorbis…

But I think that people under-appreciate my intellectual point of view.

For many months and years, I had my doubts, that MP3 Files can in fact encode ± 180⁰ phase-shifts, i.e. a stereo-difference channel that has the correct polarity with respect to the stereo-sum channel, over a range of frequencies. What my own musings have only taught me in recent days, is that in fact, MP3 is capable of ± 180⁰ phase-separation.

Further, similar types of compression should be capable of better phase-separation than that, If their bit-rates are set high enough, that not too many of their frequency-coefficients get chopped down – according to what I have reasoned out today.

What I also know, is that the sound-formats AC3 and AAC have as an explicit feature, to store surround-sound. MPEG-2 Video Files more-or-less require the use of the AC3 codec for sound, and MP4 Files absolutely require the use of the AAC codec. And, stored in its compressed format, the surround-effect only requires ± 180⁰ phase-accuracy.

This subject is orthogonal to debate which exists, about whether it is of benefit to human listeners, to have sound reproduced at very high sample-rates, or at great bit-depths. Furthermore, I do not fully know what good a very high sample-rate – such as “192kHz” – is supposed to do any listener, if his sound has been MP3-compressed. As far as I am concerned, ultra-high sample-rates have to do with lossless compression, or no compression, which also happen to produce the same file-sizes at that signal-format.

What I did was just check, in what format iTunes downloads music by default. And it downloads its music in AAC Format. All this does for me, is corroborate a claim a friend of mine made, that he can hear his music with full positioning, since that is also the main feature of AAC, and not of MP3.

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Internet TV and Bit-Rates

When we hook up our Internet TV box for the first time, we are allowed – in most cases – to set our TV resolution to 1080p. But the reality today is, that this resolution does not by itself state the picture quality.

Such video streams are sent to the receiver in compressed format, and the compressed bit-rate is a more accurate indication of what the actual picture quality will be. I.e., even if the basic format is 1920×1080, by compressing the stream more, lower bit-rates can be achieved, at the expense of picture quality.

Modern Internet TV boxes are quite intelligently programmed, to be able to change the bit-rate in mid-stream. Thus, when my ‘‘ started buffering, during an initial test, it did not only allow for its buffer to catch up with the point in the movie I was watching, but also noted that the real bit-rate of my connection was not high enough to support the highest quality level, available at 1080p. Immediately after that, the picture seemed slightly less sharp, but as I continued to watch, the overall quality of the picture started to recover again.

My has never had a streaming-interruption since then.

In fact, the way it is with HDMI connections to our TV, it is impractical for the source of the stream to change the picture-format in mid-stream. It tends to stay fixed.

And when we compare – which is a service offered by my ISP, also to stream TV to me via DSL – its reason for maintaining a consistent picture-quality is actually different. In this case, the bit-rate of the stream is reserved at the Modem / Router, which also belongs to Bell. I.e., the Bell Modem can ensure that a certain rate of bits per second are available for TV, and can do so at the expense of actual computers also trying to communicate. My is counted by the Bell Modem, as just another connected WiFi client.

What this actually means, is that if another person is considering buying a , but worried that his Internet is not fast enough – as long as he does have some form of high-speed Internet – he need not worry much. The receiver would detect his slow connection, and adjust the picture quality to suit.

Also, with a , we get to set the picture format to 720p instead, so that the required bit-rates start at a slower one.

If my TV was a 4K TV, I could set the format accordingly, but then I would worry, that this might be time and money wasted, because then, the picture quality on my network might not keep up with the 4K format.

Dirk