Apple Music with iPhone and External USB C DAC: Bit-Perfect or Altered Playback?

[Michou]

I'm having a debate with a colleague about using Apple Music on an iPhone with an external portable USB C DAC or dongle. I believe the iPhone alters or up-samples the audio in some way, resulting in a non-bit-perfect output that doesn't match the original track's sample rate.

My colleague, however, argues that this setup delivers true bit-perfect sound in exclusive mode at the same sample rate and bit depth the original track was released in.

Which one of us is correct? Does Apple Music on an iPhone with an external DAC output bit-perfect audio at the original track's sample rate, or is the audio altered or up-sampled during playback? I'd really appreciate insights from those with technical expertise!

 

[antcollinet]

I can't answer your question.

I can tell you it doesn't matter. Your debate is akin to arguing the number of angels dancing on the head of a pin.

 

[antcollinet]

who are you to judge its importance?

OK - I'll restate.

It is unimportant from the point of view of audio quality, to which it makes no difference.

If you want bit perfect for other unstated reasons, that is up to you.

 

[Michou]

OK - I'll restate.

It is unimportant from the point of view of audio quality, to which it makes no difference.

If you want bit perfect for other unstated reasons, that is up to you.

I agree that, most likely, there’s no audible difference, and this debate may seem unimportant to some. However, I’ve asked for help to settle this argument and establish the fact, primarily for my own selfish sense of resolution and clarity. There’s nothing wrong with seeking answers, especially when it helps put the matter to rest.

 

[Michou]

I can't answer your question.

I can tell you it doesn't matter. Your debate is akin to arguing the number of angels dancing on the head of a pin.

If you can’t or won’t answer my question, why bother responding at all? Your comment is a waste of bandwidth, time, and energy—for both of us. This topic matters to me, and with all due respect, who are you to judge its importance? I’ll wait for input from those who actually want to engage constructively and have something meaningful to contribute.

 

[antcollinet]

Weird. Posts out of order.

 

[Jimbob54]

Assume neither you nor your friend has a dac that displays sample rate? Thats the easiest way- see what it displays a 44.1/96khz/ file as on transfer

I dont use apple music but assume you can see on the app what the file dit depth/ SR is ?

 

[palm]

The RME ADI has a “bit test” functionality. You can find sample files on their website. The DAC will recognize the sample and display a message when successful. Importantly these samples shouldn’t harm speakers if you forget to mute.

I can confirm the test is PASS - on iOS and iPadOS it will automatically change the sampling rate (not on macOS) and there are no alterations to the signal.

 

[TunaBug]

Do you mean audio from Apple Music the streaming service? That seems like a moot question, as we have no control over what Apple did to the music between the studio and sending it to your phone. I.e. who cares if it's bit-perfect on the phone when it's probably not bit-perfect to the phone?

Or do you mean that you have your own losslessly encoded music on the iPhone that is being played back via the Music app? If the latter then it's a good question. Thank you @palm for the answer.

 

[masterhw]

Not Apple Music, but Tidal playback typically reflects the track's sample rate on my 16 Pro & Fosi DS2. If I skip a track it often reverts down to red LED (indicating 44.1/48 kHz), but allowed to rollover to the next track on its own I've seen it live switch to green (88.2/96) or yellow (...192) and back, whatever the track is.
So this hopefully indicates it is possible, but again I would think the only way to know in your situation is if the dongle has an indicator on it.

 

[Michou]

Do you mean audio from Apple Music the streaming service? That seems like a moot question, as we have no control over what Apple did to the music between the studio and sending it to your phone. I.e. who cares if it's bit-perfect on the phone when it's probably not bit-perfect to the phone?

Or do you mean that you have your own losslessly encoded music on the iPhone that is being played back via the Music app? If the latter then it's a good question. Thank you @palm for the answer.

I mean Apple Music's 'high-resolution' content when played through a recent iPhone with an external DAC or dongle DAC connected via its USB-C port (not the obsolete Lightning jack). If the DAC has a display, it should show the same sampling rate as the playing stream—so, for example, it should not display 192kHz for a 44.1kHz track, but should instead reflect the actual resolution of the audio being played.

 

[TunaBug]

I mean Apple Music's 'high-resolution' content when played through a recent iPhone with an external DAC or dongle DAC connected via its USB-C port (not the obsolete Lightning jack). If the DAC has a display, it should show the same sampling rate as the playing stream—so, for example, it should not display 192kHz for a 44.1kHz track, but should instead reflect the actual resolution of the audio being played.

Yep, got it. But to be clear, that test shows you whether or not they're changing the sample rate or bit depth, which is not the same thing as being bit perfect. They could, for example, apply some EQ in the app.

 

[Wil_Bloodworth]

Um... so if you connect an external DAC to an iPhone via USB-C, are we saying that the iPhone somehow "knows" that it should send digital data to the DAC? I always thought that if you used a USB to RCA cable that you're using Apple's internal DAC. I guess both could be true. Maybe the external DAC would be controlling or asking the iPhone for data differently from when you're just using the Apple Music app to play music to an analog destination.

I would think that an external DAC would be far superior to the one built into any iPhone.

 

[unpluggged]

iPhone somehow "knows" that it should send digital data to the DAC?

What makes you wonder here? iPhone detects a USB audio class device that has some outputs, and routes audio there. Isn't it trivial?

I would think that an external DAC would be far superior to the one built into any iPhone.

Considering the only DAC built into current iPhones is the one that drives its built-in speakers, this is a valid assumption.

 

[Wil_Bloodworth]

What makes you wonder here? iPhone detects a USB audio class device that has some outputs, and routes audio there. Isn't it trivial?


Considering the only DAC built into current iPhones is the one that drives its built-in speakers, this is a valid assumption.

It's probably trivial but I just never really thought about it before because I didn't consider it important enough to ponder.

 

[TunaBug]

so if you connect an external DAC to an iPhone via USB-C, are we saying that the iPhone somehow "knows" that it should send digital data to the DAC?

What else could it send? USB is a digital interface. It's not possible to send analog over it. That has nothing to do with iPhone or iOS.

I always thought that if you used a USB to RCA cable that you're using Apple's internal DAC.

A USB to RCA "cable" has a DAC chip inside it. You're still sending digital over USB. But your cable is not just a piece of wire: it's an active component.

 

[Wil_Bloodworth]

What else could it send? USB is a digital interface. It's not possible to send analog over it. That has nothing to do with iPhone or iOS.



A USB to RCA "cable" has a DAC chip inside it. You're still sending digital over USB. But your cable is not just a piece of wire: it's an active component.

Nice. I did not know that. Thanks!

 

[tmtomh]

In MacOS, the bit depth and sample rate of the digital audio output are fixed: you can change them using the Audio MIDI utility, but whatever you set them at, that's the output regardless of the bit depth and sample rate of whatever music file you're playing. The OS does not have "on the fly" resolution switching (though third-party apps and add-ons provide this capability).

I believe the same is true in iOS. So the output might or might not be bit-perfect - but that depends on the relationship between the bit depth and sample rate of your music file (or the Apple Music sound quality tier you select), and the bit depth and sample rate at which iOS outputs digital audio.

If your iPhone doesn't have an analogue headphone jack, then by definition you're always going to be using an external DAC to listen to music, even if that DAC is built in to your headphones or whatever.

 

[im_gumby]

I'm having a debate with a colleague about using Apple Music on an iPhone with an external portable USB C DAC or dongle. I believe the iPhone alters or up-samples the audio in some way, resulting in a non-bit-perfect output that doesn't match the original track's sample rate.

My colleague, however, argues that this setup delivers true bit-perfect sound in exclusive mode at the same sample rate and bit depth the original track was released in.

Which one of us is correct? Does Apple Music on an iPhone with an external DAC output bit-perfect audio at the original track's sample rate, or is the audio altered or up-sampled during playback? I'd really appreciate insights from those with technical expertise!

Looking thru the responses...

Ok, I have an iPhone 15 (iOS 17.7x not 18)
I have the Apple Dongle which has a DAC chip, and the ONIX Alpha XI1 (Alpha).
(Also the iBasso DX180 which is a DAP but can be used as a dongle DAC too... )

On the iPhone, when you plug in the DAC, Apple will sense that there's a device connected to the USB port and will automatically route the sound to it.
From my personal experience... the iPhone doesn't up sample. My Alpha can handle up to 32bit at 768kHz. iBasso DX180 will only go up to 32/384kHz.

On my macbook Pro, I can set the output to up sample. The Midi app will recognize the device and allow you to up sample to the limit of the device. (No options w the Apple dongle, but can set it to upsample to 32/768kHz w the Alpha.

Could you use a 3rd Party app to change the output and up sample? Maybe. Haven't tried it. Don't really recommend it either.
But that's just me.

@tmtomh is correct.

HTH

 

[palm]

On macOS indeed, but on iOS and iPadOS you can see the DAC switching sampling rate following the content, at least with Apple Music but I guess this is true for any app using standard api.

The different behavior makes sense I guess: on the mobile devices a single app has exclusivity. On the desktop I understand some might be irritated by this fixed sampling rate but it allows to set up a chain with a convolver for room correction with a FIR, and that’s just a use case for us folks, for audio professionals there a plenty of other reasons probably. Personally I am not bothered by this and rely on a good implementation of the SRC.