Re: portable CD players
- Date: Mon, 12 Nov 2018 08:35:23 -0500
- From: Greg Wooledge <wooledg@xxxxxxxxxxx>
- Subject: Re: portable CD players
On Sat, Nov 10, 2018 at 10:47:11AM -0500, rhkramer@xxxxxxxxx wrote:
> IIRC, a CD holds a maximum of 80 minutes .wav sound. (Around 800MB, about 1
> MB per minute). A 4 GB holds way more than that in .mp3, although a figure
> doesn't come to mind at the moment -- maybe more on the order of 40 hours of
> sound. And, it is reprogrammable.
For starters, your "1 MB per minute" math is off. Even using the numbers
you showed, that's 10 MB per minute.
128 kbps MP3 (a de facto standard encoding bit rate from many years ago)
is an 8:1 compression over regular CD-quality audio. This one is about
"a meg a minute". At least as a first-order approximation.
So, a gigabyte of 128 kbps MP3 audio data is about a thousand megs,
therefore about a thousand minutes, or 17 hours. At 4 GB, you're looking
at about 67 hours.
Of course, not all audio data is compressed at a bit rate of 128 kbps.
In more recent years, with storage and bandwith prices dropping, people
have started using much higher bit rates on music files. It's not
at all uncommon to see MP3 encoded at 256 kbps or higher these days.
That drops you to 33 hours for a 4 GB storage device.
If your music library contains a mixture of songs encoded at varying
bit rates, you'll probably be somewhere in between these values.
And not all audio data is music. Human speech can be compressed at a
much lower bit rate than music. A quick google search tells me that
64 kbps is common for audio books. That bumps you up to 133 hours, if
you're storing MP3 audio books. Possibly more, since 64 kbps seems to be
an upper bound on speech encoding.
There are also dedicated speech encoding algorithms. Speex was one of
them, but I believe it never really caught on. I think it was obsoleted
by the Opus codec, which is starting to catch on a little bit.
According to <https://wiki.xiph.org/index.php?title=Opus_Recommended_Settings>
the recommended Opus bitrate for audio books is 24 kbps for mono, or
32 kbps for stereo.
The problem with Opus is that a hardware player may not support it.
The most recent hardware player that I've got (several years old) does
not support Opus, but it does support Ogg Vorbis, MP3, WMA and other
formats that have been around for decades.