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I am surprised that Skype is ranked better than a wired phone.
by lqet 6y ago
I am surprised that Skype is ranked better than a wired phone.
- WalterBright 6y agoIt has a higher bit rate.
- tremon 6y agoA wired phone (POTS) doesn't have "bit rate", it has bandwidth. It could very well be that VOIP uses a lower bitrate than Skype, or Skype uses a better algorithm, but I would lump VOIP in the "Skype" category, not the "wired phone" category.
- WalterBright 6y agoI do know that POTS is analog (at least the wire to my house is, I presume at some point it becomes digital), not digital, but the larger point stands.
- mnw21cam 6y agoA wire phone (POTS) definitely does have a "bit rate". The entire POTS back-end system is digital, and each circuit gets 8000 8-bit samples per second, or 64kbps, uncompressed.
- msla 6y agoAnything with a bandwidth has bitrate: https://en.wikipedia.org/wiki/Nyquist%E2%80%93Shannon_sampling_theorem https://en.wikipedia.org/wiki/Nyquist%E2%80%93Shannon_sampli...
- tremon 6y agoWell, to put it simply, no. Nyquist's theories (and Shannon's sampling theorem based on it) only concerned themselves with the time-based discretization of analog signals. As Shannon showed, to accurately reconstruct a signal of bandwidth x, you need to take samples at a constant frequency of 2x. But when speaking of digital signals, discretization is only half the story. The other part of the story is quantization, i.e. how many bits to use to represent each sample. And that amplitude quantization is a lossy operation, regardless of Shannon's theorem about discretization in the time domain. You can't simply say that an analog signal of 40kHz therefore has a bitrate of 80kbps -- using just one bit to represent each sample will not accurately reproduce the original analog waveform, unless the original waveform was a square wave (and you would still need that bit of information to accurately reconstruct it). So what, then, is the bitrate of a discretized, analog signal? It is still infinite, because the amplitude of every sample can be any element of R! Only once you decide on a quantization resolution, i.e. once you determine your acceptable level of signal loss, can you speak of "the bitrate of an analog signal". But that bitrate is only an approximation of the original signal, Shannon notwithstanding.
- msla 6y agoYour flat dismissals do nothing to disprove Shannon's math. Come back when you have equations.
- lqet 6y agoI am not so sure: according to Wikipedia, Skype uses SILK, which has bitrate of 6-60 kbit/s [0]. In contrast, G.711 (often used for classic landline telephones) has a bitrate of 64 kbit/s [1]. I can only speak from personal experience: my cheap standard landline has much better audio quality and lower delay times than any Skype call I have made. [0] https://en.wikipedia.org/wiki/SILK https://en.wikipedia.org/wiki/SILK [1] https://en.wikipedia.org/wiki/G.711 https://en.wikipedia.org/wiki/G.711
- Tsiklon 6y agoIs this bitrate comparison an "apples to apples" comparison? I know when talking about audio compression in the frame of music reproduction AAC 256Kb/s is similarly performant to MP3 320Kb/s - and I think there are better performing audio codecs still.
- EVdotIO 6y agoBitrate only tells a little bit of the story. There are high pass and low pass filters, audio compression, data compression, latency, error correction and a number of other things at play with these codecs to hit a desired bitrate with the goal of maximizing for highest legibility over unreliable connections. This leads to a number of compromises; in the case of Skype this means you can have huge variance in quality, and landlines are consistent but incredibly limited in reproducible bandwidth and dynamic range.