Understanding iPhones/iPods

[cukaracha]

When an iPhone/iPod is connected to a compatible receiver via USB, how does it send audio through?

On older iPods sporting the 30-pin connector, there are line out connections on the connector, but these aren't connected to the USB end of the cable. This would imply that audio signals are sent digitally.

Here's where I get confused:

- audio receivers would typically expect a mass storage device when a USB connection is established, meaning it looks for audio files. In that case, how do streaming services such as Spotify work?

- since iPods do not act as storage devices, does this imply that all audio is streamed digitally?



I really hope someone can help clarify this for me.


Thanks!!

 

[scopeboy]

Many Apple devices actually support USB audio. It used to be possible to plug a USB DAC into the IPhone/iPad camera connection kit. So, the receiver probably just looks like a USB soundcard to the iDevice.

 

[nigelwright7557]

There are two ways USB audio works.
1/ Device acts like flash drive to hold audio files.
2/ Device converts immediately digital data to analogue data.

I have a MP3 player that does (1).
I have a NDX900 USB player that does (2).

 

[cukaracha]

Thanks for the replies.

I can see how the iPod would see the receiver as some soundcard, but how does the receiver distinguish an iPod from a flashdrive?

 

[planet10]

In Apples developer infothey may have the specifics. If it is out via USB it will be as if you were using a computer to feed your DAC via USB.

dave

 

[scopeboy]

Thanks for the replies.

I can see how the iPod would see the receiver as some soundcard, but how does the receiver distinguish an iPod from a flashdrive?

It is not even the same way round. The phone is the USB host, the receiver is the USB device.

Most phones have "USB On The Go" which can work either way round depending on the status of a selector pin. My Android phone appears as a USB device to my computer, but using an OTG adaptor lead I can plug a flash drive or USB audio DAC into the phone.

Apple probably have USB OTG too, just hidden in a proprietary connector and augmented with a secret chip that makes it only talk to Apple approved devices.

If the receiver can accept a flash drive and an iPod in the same USB port, and transfer audio digitally, then it must have some sort of OTG functionality too.

 

[cukaracha]

Thanks again for all the replies.

My Android phone appears as a USB device to my computer, but using an OTG adaptor lead I can plug a flash drive or USB audio DAC into the phone.

If the receiver can accept a flash drive and an iPod in the same USB port, and transfer audio digitally, then it must have some sort of OTG functionality too.

Interesting. In that case, does the receiver distinguish from mass storage and OTG via impedance (or some other physical electrical quantity), or by communicating with the device? I know that Arduinos have Apple Accessory libraries that allow it to be recognized as an Apple accessory, so I'm guessing that the communication protocol is what cues the receiver?