CD-to-MP3 ripping speed estimation

As every sensible car-owner in Brussels, I rip my CDs to MP3 so I can put copies of them in my car. As every self-respecting geek, I have multiple PCs at home. Which brings me to following observation: not all PCs rip alike. On one PC the CPU maxes out at 100% for the whole ripping procedure, and on the other, I never get above 75%. So I started wondering: what are the elements to define the maximum ripping speed you can get on a PC?
My hunch:

the CD-ROM drive speed:

the original CD audio specification required a constant data rate. This was implemented by running the CD at 500 rpm for the first/inner tracks on the CD (ø 48mm) and at 200 rpm for the outer tracks (ø 118mm). If the CD would have been played at a constant 500 rpm, the data rate at the end would have been 500/200 = 2,5X. (cf Devnulled: Ripping speed)
With CD-ROM the data should be delivered as fast as possible. So the rotation speed is turned up as much as possible. The physical boundaries are the vibrations and the centrifugal forces that occur at high speeds. Maxwell claims the maximum safe speed is 48X. Since the “48X” is marketing speak, this speed is only obtained at the outer border of the CD: this means that the rotation speed would be 48 x 200 = 9200 rpm. Some CDs seem to explode above 10.000 rpm.
To convert this speed into a data rate: at 9200 rpm, the outer tracks would deliver 48x the data rate of an audio CD: 67,74 Mbps or 8.47 MB/s. The first tracks, at ø 48mm, deliver data 2,5 times slower: 27,52 Mbps or 3,44 MB/s.
Real-life tests of a whole bunch of drives on DAE speed results.
For the exact sizes: CD-R/CD-RW technical specifications

the bus speeds:

the CD-ROM drive is connected to the PC by a ATAPI, SCSI, FireWire or USB connection. In theory there could also be a network in between (e.g. when using a Ethernet connected CD Jukebox).
The slowest ATA-33 has a theoretical max throughput of 33MB/s. Most modern SCSIs go above 20MB/s and FireWire gives 50 MB/s. So they would not be the bottleneck in the ripping process.
USB1.1 is limited to 1,5 MB/s (in practice even lower). Most common networks would be a bottleneck too (even Fast Ethernet at a theoretical 12,5 MB/s since 7MB/s would be more of a realistic top rate in practice, certainly if the network is used for other stuff too. Same thing with WiFi standards: 802.11g’s advertised “54Mbps” will in real life never translate in an actual 6,75MB/s throughput.

the CPU speed:

encoding raw audio data to MP3 is CPU intensive. Main parameter will be the clock speed – which I would expect to scale linearly: a 2GHz processor does it twice as fast as a 1GHz. Extra influences: brand of processor (Intel/AMD), model (Celeron/Pentium4/Athlon/Athlon64), number of processors (or HyperThreading). Also, the software you use to encode (LAME/GOGO/RealPlayer/Windows Media Player/…) will have an impact.
Some data can be found on GamePC.com: an Intel P4 3.06 GHz encodes 200MB of raw data info 160 kbps MP3 in 57 seconds: 3,5 MB/s or 20X. The AMD AthlonXP 2700+: 3,28 MB/s or 18.6. More info on GamePC.com confirms our hunch that performance scales linearly with clock speed. For the Pentium4: (1,15 MB/s) per GHz or 6,5X per GHz.

the MP3 bitrate:

the above numbers are for 160 kbps, but what with 192 kbps and 64 kbps? Is encoding faster or slower? I found no data on the net, and I haven’t tested it myself. So no hunch here. Also, the output of the encoding process, even at a very high quality 320kbps is largely within the capacity of any output, even Bluetooth, god forbid. So I don’t take that parameter into account.


So in the following situation:

  • a 24X CD-ROM drive
  • a Pentium 4 2,8GHz processor
  • ripping with the LAME encoder to 160 kbps

Your ripping will start at about 9,8X and speed up until your CPU is saturated at 18,2X. Which gives the graphic at the right. Now there’s a rule of fist.

Remark: looking at the benchmarks, adding a second processor (or HyperThreading) does not enhance the ripping speed (probably since the MP3 encoding code does not do parallelisation). But if you have 2 CPU’s, only one CPU will go to 100% and you keep some breathing room while your PC is creating the MP3s.

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