An Observation (Perhaps an Obvious One) About Encoding Speeds

[chucknorris]

I noticed today, while converting some lossless files into AAC that iTunes encodes noticeably faster from the hard drive (in this case 7200 RPM Lacie from sig) than from cd. Though this seems quite intuitive to me, it got me thinking.

Is the bottleneck the combo drive, the ATA bus, a little of both, or something else? If it's the drive, how do PowerMacs manage so much faster encode times (yes, I do know the drive is faster)?

I'm sure these questions are incredibly stupid, but curiosity cannot wait.

 

[eXan]

I am also very interested in why CD->160Kbit AAC is sometimes encoding at 5x-6x speeds and sometimes 7x?

(no background apps taking more than 2% of CPU (checked in Activity Monitor))

 

[mad jew]

The speed can depend heavily on how hot your machine is running, what speed the processor performance is set at and how scratched the CD is, among many other things of course.

 

[Mr_T]

I could be wrong but I believe the bottleneck happens at the CD-Drive level. A drive reading from a clean CD at 40x would get roughly 5.8MBps for the transfer rate. Even slow hard drives are much much faster than this, starting at 33MBps. This is why the Optical drives on Macs are usually attached to an ATA33 channel - that's more than enough bandwidth. Of course dirt and scratches slow it down even further.

I always assumed the speed changed (say starting at 5x and then ending up around 7x when encoding in iTunes) when encoding a disc because the further the laser got to the outer edge of the CD, the faster the disc was spinning and the faster the data could be read, could somebody tell me if this is right?

~Mr.T

p.s. also, if the planets are aligned right and you say the magic words, it goes much faster.

 

[punkbass25]

nope! fairly certain that the speed of the disc has little to do with it.now a cd drive in a computer might be diffrent. but my old cd player would turn the disc slower as it got to the outside, to compensate for relative speed of the disc where the laser is. so that the cd is always moving the same speed from the lasers point of view...

 

[homerjward]

the bottleneck is processor power. that's why you get faster on your g5.

 

[enginerd]

CD and CPU speed should be the biggest factors, specfically digital audio extraction (DAE) speed. Vanilla CD-RW drives (not combo, dvd-rom,dvd-rw,etc) tend to have the fastest CD read and DAE speeds. Before these drives began to disappear, they topped out at 52X max. Most only reached that speed at the edge of the disc though. With my old 48X cd-rw on my PC, I could rip cd's at 24-40X speed - most CDs under 4min. But now with my dvd-rw drive, it's a lot slower because it's cd read speed is only 32x and the DAE speed is even less. (I don't rip cd's much now so I never timed it now)

The PM's are probably helped more by the faster cd read speeds than their faster processors. All of the other macs use laptop optical drives and they probably top out at 24X speed (except for emac, but not sure how fast that one is)

 

[mkrishnan]

I think dynamic balance (physically, how flat the disc is) has a lot to do with it too... small amounts of warping in the CD can make it unstable at speed and cause the CD drive to spin down to a slower speed. I've also always suspected that the full (1/2 height) size CD drives also are able to spin a CD with the same level of dynamic balance faster because they can anchor it more tightly than the slot-loading drives...but I don't have any data to back that up.