GitHub - Bloomca/rust-cd-da-reader: Library to read track...

This is a simple library to read audio CDs. At the core it was written to enable CD ripping, but you can also implement a live audio CD player with its help. It is cross-platform and tested on Windows, macOS and Linux and abstracts both access to the CD drive and reading the actual data from it. All operations happen in this order on each platform:

Get a CD drive's handle
Read ToC (table of contents) of the audio CD
Read track data using ranges from ToC

Let's go through each concept in order.

CD access

First thing, we'll need to get a hold of the CD drive. You can see the drive's letter on Windows in File Explorer (although the actual handle will be something like "\\.\E:"), with cat /proc/sys/dev/cdrom/info on Linux and with diskutil list on macOS.

This is a bit brittle, so this library provides a few helper methods to find a correct CD drive. By far the most straightforward approach is to simply open the "default" drive:

use cd_da_reader::{CdReader};

let reader = CdReader::open_default()?;

This code will scan the CD drives and will open the first one with an audio CD in it, and usually this is what you want. If you want to provide a choice, there is an additional function to list all drives:

use cd_da_reader::{CdReader};

let drives = CdReader::list_drives()?;

This will give you a vector of drives, and the struct will have has_audio_cd field for audio CDs. Unfortunately, this does not work on macOS due to how CD drive handles are treated. When we execute any command to a CD drive (which we need to check whether the CD is audio or not), we need to claim exclusivity, which will cause it to unmount. If we release the handle, it will cause it to remount, and that will do 2 things:

call the default application for an audio CD (probably Apple Music)
that app will claim exclusivity, so we won't be able to get it back for some time

Because of that, on macOS you should either provide the name by yourself, or get the default drive:

use cd_da_reader::{CdReader};

// get the default drive, which should be what you want
let reader = CdReader::open_default()?;

// or read the disk directly
let reader = CdReader::open("disk14")?;

Reading ToC

Each audio CD provides internal Table of Contents, which is an internal map of all the available tracks with the block addresses. The only semantic metadata we get from it is the number of tracks, but it is crucial to read it so that we can issues commands to read actual tracks data.

use cd_da_reader::{CdReader};

let reader = CdReader::open_default()?;
let toc = reader.read_toc()?;

This will give us a struct like:

{
    first_track: 1,
    last_track: 11,
    tracks: [{
        number: 1,
        start_lba: 0,
        start_msf: (0, 2, 0),
        is_audio: true,
    }, {
        number: 1,
        start_lba: 14675,
        start_msf: (3, 15, 50),
        is_audio: true,
    }, ...],
    leadout_lba: 221786
}

LBA (Logical Block Address) is a simple sequential sector index. LBA 0 is the first readable sector after the 2-second lead-in pre-gap at the start of every disc. It is the most convenient format for issuing read commands and used internally to read data blocks.

MSF (Minutes:Seconds:Frames) is a time-based address inherited from the physical disc layout. A "frame" here is one CD sector, and the spec defines 75 frames per second. MSF includes a fixed 2-second (150-frame) offset for the lead-in area, so MSF (0, 2, 0) corresponds to LBA 0 — the very start of track data.

The two are fully interchangeable: LBA + 150 = total frames from disc start, from which minutes, seconds, and frames are derived by dividing by 75 and 60. You will typically only need LBA values for reading track data, while MSF is required for services like MusicBrainz disc ID calculation.

Reading tracks

Finally, after we got ToC, we can read tracks. The boundaries for the track are the starting LBA and the starting LBA for the next track (or leadout LBA value for the last track). This library abstracts these things and simply reads provided track numbers. To read a track, all you need to do is call:

use cd_da_reader::{CdReader};

let reader = CdReader::open_default()?;
let toc = reader.read_toc()?;
// we assume that track #1 exists for simplicity
let data = reader.read_track(&toc, 1)?;

This is a blocking call and takes a lot of time (depends on the track length and CD/drive quality due to retries). If you want to do something with the data as it comes, use streaming API:

use cd_da_reader::{CdReader, RetryConfig, TrackStreamConfig};

let reader = CdReader::open_default()?;
let toc = reader.read_toc()?;

let stream_cfg = TrackStreamConfig {
    sectors_per_chunk: 27, // ~64 KB per chunk
    retry: RetryConfig::default(),
};

let mut stream = reader.open_track_stream(&toc, 1, stream_cfg)?;
while let Some(chunk) = stream.next_chunk()? {
    // do something with the chunk directly
}

Track format

The data you receive by reading tracks is PCM, the same raw format used by WAV files. Audio CDs use 16-bit stereo PCM sampled at 44,100 Hz, so each second of audio is:

44,100 samples * 2 channels * 2 bytes = 176,400 bytes/second

Each CD sector holds exactly 2,352 bytes of audio payload (176,400 / 75 = 2,352), that's why there are 75 sectors per second. A typical 3-minute track is about 31 MB of raw PCM, and a full 74-minute CD holds ~650 MB.

Converting PCM data to a playable WAV file only requires prepending a 44-byte RIFF header. In fact, there is a helper for that in this library:

use cd_da_reader::{CdReader};

let reader = CdReader::open_default()?;
let toc = reader.read_toc()?;
// we assume that track #1 exists for simplicity
let data = reader.read_track(&toc, 1)?;
let wav = CdReader::create_wav(data);
std::fs::write("myfile.wav", wav)?;

This code will read the first track from the CD file and save it as a WAVE file, which will be playable by any music player.

What about metadata?

You might have asked why do we expose LBA/MSF values if the track reading is abstracted behind specific track numbers. The reason for that is metadata. Even though there is a command CD-TEXT for storing data directly, it is not exposed in this library due to it being extremely unreliable.

Instead, you can calculate a Disc ID for a service like MusicBrainz, which requires full ToC for it: ref. You can see an example of how to calculate the ID here.

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