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Curious Difference on Processing Apple Lossless vs. AIFF


cscuzzo

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I've been tweaking my new Mac mini installation. It uses an SSD for the operating system disk, with a firewire external drive for music. I've kept the Activity Monitor running continuously to make sure I understand what is running, whether it should be, etc.

 

All of my music is encoded in Apple Lossless from the time I ripped the discs 6 months ago. A friend of mine has been teasing me that I need to switch to AIFF for sonic reasons. So I converted one disc to AIFF (a good reference disk) for a comparison. Well, the difference was extremely slight--so much I am not sure it exists at all.

 

However.....suddenly the disk activity on the Activity Monitor went from a continuous read rate of 60-80 KBytes/sec down to nearly zero. So it appears that external drive traffic doesn't count as "disk activity" in that application. So what is OS/X doing generating over 60Kbyte/sec disk traffic (on the OS disk, presumably) when processing Apple Lossless?

 

(Edit: There's some funny business going on with Activity Monitor. I'm now doing a conversion from Lossless to AIFF for the whole library, and it's reporting all sorts of disk activity that my theory wouldn't support. Not sure what's going on here.)

 

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I think the answer is that Activity Monitor and "top" are not reliable indicators of disc activity. Using top I was seeing the similar behaviour accessing the internal drive with the difference that ALAC was showing zero activity and AIFF high levels of read activity. Clearly this is incorrect as any media player has to read the original file from disc at some point, and the large the file the more data needs to be read.

 

I ended up looking at disc activity using iostat -dC -w1.

 

 

iostat -dC -w1

disk0 cpu

KB/t tps MB/s us sy id

0.00 0 0.00 2 6 92

50.00 10 0.49 11 9 80 < Start ALAC Playback

68.00 1 0.07 5 9 86

68.00 1 0.07 3 8 89

68.00 2 0.13 4 9 87

68.00 1 0.07 3 8 89

68.00 1 0.07 4 8 88

68.00 1 0.07 4 8 88

68.00 2 0.13 3 9 88

68.00 1 0.07 4 8 88

disk0 cpu

KB/t tps MB/s us sy id

68.00 1 0.07 3 8 89

68.00 2 0.13 8 10 82

68.00 1 0.07 4 9 87

68.00 1 0.07 4 8 88

68.00 1 0.07 4 8 88

15.91 23 0.36 3 11 86

68.00 2 0.13 4 8 88

68.00 1 0.07 3 8 89

68.00 1 0.07 4 8 88

68.00 2 0.13 3 9 88

0.00 0 0.00 4 8 88

54.67 3 0.16 8 11 81

68.00 2 0.13 5 9 86

68.00 1 0.07 4 8 88

68.00 2 0.13 3 9 88

68.00 2 0.13 3 8 89

68.00 1 0.07 4 8 88

68.00 2 0.13 3 9 88

68.00 1 0.07 4 8 88

68.00 1 0.07 4 8 88

disk0 cpu

KB/t tps MB/s us sy id

68.00 1 0.07 3 8 89

68.00 2 0.13 7 10 83

68.00 1 0.07 5 9 86 < End ALAC playback

0.00 0 0.00 2 7 91

0.00 0 0.00 5 7 88

0.00 0 0.00 2 6 92

0.00 0 0.00 2 5 93

0.00 0 0.00 4 6 90

56.25 16 0.88 10 11 79 < Start AIFF playback

68.00 3 0.20 2 8 90

68.00 3 0.20 3 8 89

68.00 2 0.13 2 8 90

68.00 3 0.20 3 8 89

68.00 3 0.20 2 9 89

68.00 2 0.13 3 8 89

23.07 15 0.34 3 10 87

68.00 3 0.20 2 8 90

68.00 2 0.13 61 15 24

692.80 20 13.52 40 16 44

68.00 3 0.20 4 9 87

disk0 cpu

KB/t tps MB/s us sy id

68.00 3 0.20 3 8 89

68.00 2 0.13 3 9 88

68.00 3 0.20 2 8 90

68.00 3 0.20 3 8 89

68.00 2 0.13 2 8 90

68.00 3 0.20 3 8 89

68.00 3 0.20 3 9 88

68.00 2 0.13 2 8 90

68.00 3 0.20 7 10 83

31.43 7 0.21 3 9 88

68.00 2 0.13 3 8 89

68.00 3 0.20 3 8 89

68.00 3 0.20 2 8 90

68.00 2 0.13 3 8 89

68.00 3 0.20 2 9 89

53.33 3 0.16 3 8 89 < End AIFF Playback

0.00 0 0.00 3 8 89

 

The columns are: Kilobytes per transfer (KBt), transfers per second (tps), Megabytes per second (MBs), CPU usage: user, system and idle.

 

AIFF playback shows a higher number of transfers per second and a higher amount of data transferred per second. On the flipside ALAC playback shows a 1-2% higher level of CPU utilisation, which should be related to decompression of the ALAC files.

 

cheers

Paul

 

 

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