Re: iPad USB Audio Class 1 and Update on OSX Class 2
Re: iPad USB Audio Class 1 and Update on OSX Class 2
- Subject: Re: iPad USB Audio Class 1 and Update on OSX Class 2
- From: Gordon Rankin <email@hidden>
- Date: Tue, 13 Apr 2010 15:08:54 -0400
- Organization: Wavelength Audio, ltd.
William Stewart wrote:
4) I take it that self powered Class 1 Audio devices could be connected directly to the the Camera Connection Kit?
if they draw less than 100mA, otherwise you need a powered hub.
Bill, Self Powered devices are not attached to the VBUS signal and
therefore do not require any power from the HOST controller. Some that
are, only use that line to indicate connection to the host. Others like
some of mine leave that off in hopes of lower noise connection from the
host.
Bill of course the first thing I did when I saw this was plug in my iPad to a powered hub as the host and then put my Proton dac into one of the available down ports and ran iTunes.... oh well no audio or stream.
please file a radar. we've done a preliminary sweep of devices.
Bill the Proton is 24/96 Asynchronous USB Class 1 device at Full Speed.
I can recompile my code for 16 bits and limit the output to 44.1/48 and
retry this. The dac uses a lithium ion powered dac chip with analog
(digitally controlled) volume control. So it does support volume L/R and
Mute which would be the basic USB Feature Unit.
So from what you are saying the iPad into the Host Port of any USB HUB
should allow connection to a downstream USB Class 1 device that is 16
bit and does not exceed 48KHz sample rate.
I worry a little bit about the asynchronous USB support as that would
require the iPad to support data throttling via another endpoint that is
linked to the primary endpoint. For instance here is partial output of
the Proton's enumeration as saved off via the USBProber:
Audio Class Specific Audio Data Format
Audio Stream Format Type Desc.
Format Type: 1 PCM
Number Of Channels: 2 STEREO
Sub Frame Size: 3
Bit Resolution: 24
Sample Frequency Type: 0x04 (Discrete)
Sample Frequency: 44100 Hz
Sample Frequency: 48000 Hz
Sample Frequency: 88200 Hz
Sample Frequency: 96000 Hz
Endpoint 0x01 - Isochronous Output
Address: 0x01 (OUT)
Attributes: 0x05 (Isochronous asynchronous data
endpoint)
Max Packet Size: 588
Polling Interval: 1 ms
Class-Specific AS Audio EndPoint - Isochronous output
Attributes: 0x01 Sample Frequency,
bLockDelayUnits: 0x00 (UNDEFINED)
wLockDelay: 0
Endpoint 0x83 - Isochronous Input
Address: 0x83 (IN)
Attributes: 0x01 (Isochronous no synchronization
data endpoint)
Max Packet Size: 3
Polling Interval: 1 ms
The Endpoint 0x83 is an input, let's call it flow control packet sent to
the host to allow more, less or even samples to the 0x01 Endpoint which
is the main output pipe.
The obvious benefit of Asynchronous USB Class 1/2 is extremely low
jitter at the output dac as fixed oscillators can be used at the dac
chip and also to derive the serial audio protocol (I2S, L/R Justified,
DSP, etc...).
Thanks
Gordon
--
J. Gordon Rankin
Owner and Chief Scientist
====== Wavelength Audio, ltd ======
_______________________________________________
Do not post admin requests to the list. They will be ignored.
Coreaudio-api mailing list (email@hidden)
Help/Unsubscribe/Update your Subscription:
This email sent to email@hidden