Subject: Re: [linux-audio-dev] Introducing DMIDI
From: Dominique Fober (fober_AT_grame.fr)
Date: Wed Dec 19 2001 - 19:01:43 EET
>> from my point of view, preserving the transmitted events scheduling with a
>maximum accuracy is important as this scheduling is part of the musical
>information itself. However, except considering that the protocol is
>intented to run on a fast local dedicated network, the transport latency
>will introduce an important time distortion. Therefore, a mecanism to
>compensate for the latency variation seems to me to be necessary.
>
>RTP timestamps are used for this.
Sure, it may be used for this but it isn't: MIDI events are played at reception time. In MWPP for example, the timestamp is only used to determine wether a packet is too late or not.
>
>> Another point is the efficient use of the transmitted packets: sending one
>packet for each event is probably not the best solution. In this case and
>due to the underlying protocols overhead, the useful information part of a
>packet may become less than 10% of the packet size. Moreover, hardware
>layers such as Ethernet for example, often require a minimum packet size to
>operate correctly.
>
>I think a protocol for realtime MIDI over UDP will always have significant
>protocol overhead, I
>don't see this as a problem however.
Considering a 44 bytes overhead (IP + UDP) + the 4 DMIDI header bytes intended to address a specific node and device, sending a full MIDI data flow (about 1000 3-bytes events per second) requires nearly 400 kbs when the MIDI rate is 31.25 kbs. It's not a problem as long as the corresponding bandwith is available to you. But if you plan to address different devices on the same node (for example using a multiport interface), you should be able to provide each device with an equivalent full MIDI data flow and then the problem seriously increases with the number of devices.
--df
----------------------------------------------
Dominique Fober <fober_AT_grame.fr>
----------------------------------------------
GRAME - Centre National de Creation Musicale -
9 rue du Garet 69001 Lyon France
tel:+33 (0)4 720 737 06 fax:+33 (0)4 720 737 01
http://www.grame.fr
This archive was generated by hypermail 2b28 : Wed Dec 19 2001 - 18:58:20 EET