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: William Stewart <email@hidden>
- Date: Tue, 13 Apr 2010 13:16:36 -0700
On Apr 13, 2010, at 12:08 PM, Gordon Rankin wrote:
>
> 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.
Yes, good point :)
>>
>>> 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.
yes (this will change as we improve/add features, but this is the base level support we have in now)
Bill
>
> 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 ======
>
>
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