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Just What IS the Signal Flow Starting From iTunes Song/Album Selection To Output?


Mister Wednesday
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Hello all,

 

This might seem like a REALLY basic question to all you computer experts out there but it's something this computer newbie has always wondered about.

 

From the instant you select a song/Album from your iTunes library listings where exactly and in what order does the data file flow through the computer system? Does it go from your drive storage location to RAM and then to cache and does it go as a single complete file or does it go through a flow out/replenish/flow out cycle to the final output point of your computer? And at each point in its flow through the system what are the important specs of each piece of circuitry that it flows through - for example, does it count to have as much RAM as possible or cache, etc?

 

Just wondering,

 

Mister Wednesday

 

 

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an interesting extension of this 'exercize' might be:

 

1) Assume that the audio files are stored on an external disk accessed wirelessly (a la Airdisk function of Airport Extreme)

 

2) Assume playback wirelessly via Airport Express or some other Airtunes client (e.g. Apple TV).

 

 

 

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cfmsp,

 

Actually That's pretty close to how my system is set up. I've got a Drobo external drive connected diretly via a Firewire 800 connection to my MacBook Pro and feeding the audio output signal wirelessly through an Airport Extreme to an Airport Express connected by a Toslink optical cable to my Linn Kisto's onboard DAC.

 

So if anyone wants to explain the signal flow chain from the external drive to the Express be my guest! But my original question concerning the signal flow through the computer circuitry still stands too.

 

Regards,

 

Mister Wednesday

 

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There was a really extensive thread a month or two ago, where I believe Axon gave a good rundown. From memory (and not, sadly, from expertise) the gist of it was this: it's all data that gets passed around asynchronously and cached multiple times until finally, the soundcard applies a clock to it, so theoretically (and likely in practice too, until shown otherwise) everything that happens before then is irrelevant. In your case, it's all data until it gets sent in asynchronous packets to the AEx, which converts it to .wav, puts it in a buffer, and sends the bits out one by one according to its onboard clock, via spdif, to the Kisto. Thus with the Linn DS system, the only jitter is whatever intrinsic jitter is in their implementation of the clock. Neat.

 

The whole process was something like this for a non-wireless implementation: the jukebox (itunes) calls for the data, the CPU calls from it from the NAS, the Nas controller reads from the platters into a cache. The NAS controller sends the data from the cache in packets asynchronously it to the CPU which sends it to RAM where the player decodes it into .wav format and sends is to the CPU which directs it to the soundcard which reads it into a buffer and then spits out the bits according to its own clock.

 

To answer your question more specifically, I think it reads a bit at a time, and as long as it is not running out of data and hiccuping, I don't see how the cache size would matter.

 

Apologies if that is wrong in any way, but I'm pretty sure that's about the size of it.

 

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This is a very interesting thread, but I'm not even going to attempt to answer the original question. The more I know the more I realize I don't know. I'm not sure we could get a group of experts to agree on what exactly happens. Plus, different computers, operating systems, and playback applications may all work differently.

 

I'd love to read more info on this one!

 

Founder of Audiophile Style

Announcing The Audiophile Style Podcast

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I was just looking at Activity Monitor as I played various file in iTunes.

 

With an AAC file there was very limited disk access, significantly more when playing an ALAC or AIFF file. With AAC there is a burst as you skip to the next track. I assume this means that iTunes read as much as possible of the file into memory - so would be a good reason for having extra memory.

 

My Mac has 2GB with a significant active VM / swap file listed (nearly 40GB - seams wrong). The fact that such a large amount of VM is used may explain how a SSD improves sound quality as SSD access should be faster than physical disk.

 

Just an observation, doesn't really answer the original question.

Eloise

 

Eloise

---

...in my opinion / experience...

While I agree "Everything may matter" working out what actually affects the sound is a trickier thing.

And I agree "Trust your ears" but equally don't allow them to fool you - trust them with a bit of skepticism.

keep your mind open... But mind your brain doesn't fall out.

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usernaim250 and Audio_ELF,

 

Thanks for your summaries of your thoughts on what's happening inside the computer when using iTunes to stream music. Very much appreciated as always and it's getting me closer to getting a basic understanding of how this all works.

 

I'm trying to get this basic understanding just for general knowledge but also to understand what are the most critical parts within the system (I guess though it's ALL critical when looking at it as an integrated system) and, if upgrades were able to be made, where you would get the most bang for your buck if you could upgrade something within a computer.

 

Audio_ELF,

 

I never considered the VM aspect of the system until you mentioned that in your reply. I'll have to take a look within my own MacBook Pro to see how mine is spec'd out in that regard. Thanks for pointing me in that direction in this research. When I bought my computer I had them upgrade the RAM to 8 GB so that I wouldn't have to worry about that for a while so it sounds from your reply that having extra RAM headroom is always a good thing for music streaming.

 

usernaim250,

 

Just a minor correction - the Linn Kisto is their A/V controller/pre-amp as distinct from their DS line of digital streaming players. I was curious as to why music from my computer system sounds better playing through the Kisto's onboard DACs than the same CD when played though my Linn Unidisk 1.1 player so I asked the engineers at their head office in Scotland about this. The engineer advised me that the Kisto utilizes a AD1896 sample-rate converter which is used to de-jitter the clock from the SPDIF / Toslink digital input - the de-jittered clock is used for the Kisto's DACs. So that explanation from Linn fits in perfectly with your explanation too. Thanks for that.

 

Regards to all,

 

Mister Wednesday

 

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