Gsoc2010 Dvs Mode

Adding a digital vinyl system (DVS) / external mixer mode to Mixxx (GSoC 2010 project)

• Student: Bill Good
• Mentor: Adam Davison

Abstract

Adding an external mixer mode to Mixxx will greatly improve Mixxx's ability to act as a part of a digital vinyl system. Mixxx's current method of achieving this involves diverting one deck to the main output, and the other to the headphone output, allowing the two to be mixed externally. However, no features, such as the equalizer, can be disabled. Benefits of disabling various features include increased CPU time for other threads, and UI recognition of unwanted features.

More info

• dvs_mode has some good ideas, as does GSOC2010 page.
• I'm bkgood on launchpad and freenode
• To contact me by email, use my irc name @gmail.com

Current status

I'll update this at least weekly, to make sure I'm keeping up with what needs done. May do a blogger on this too, never have done a blog before though. I'm going to try to include _everything_ mixxx-related I do so on days where I dink around with something other than my project I know I didn't totally waste the day :)

• sometime prior to 20100601: Digging through the mixxx code base to try to further figure out implementation details, plus learning a bit about how sampling (digital representation of analog signals) works and skimming a book on Qt.
• 20100601: Added preferences option to dlgprefsounddlg.ui for external mixer. Based it on QStackedWidget, so we just move though the stack to select between internal/external mixer devices.
• 20100602: Read through features_hydra merge diff. Looks like a step in the right direction (lots of silly stuff gone from mixxxapp::mixxxapp, yay!). Going to write up thoughts on preferences diag. Preferences diag limited in size so mixxx can run and be useful on netbooks. Would really like to make the audio preferences (and ideally all preferences) such that settings would be reverted if user doesn't click "OK." Would also like to make a "test" button to send either white noise or a sine wave to a specific channel pair on a device so that the user can be sure he's got the right channel going to the right mixer (or amp) input. dlgprefsound.cpp is a bloody mess, can't wait to go after it once I've got the dialog looking like I want it.
• 20100609: Quick update as I'm tired as hell: decided (final change of heart/mind, I promise) that I didn't want to start from the top (ie., UI) down and I'd much rather just build a nice and pretty backend, so I uncommitted my UI file change and finally figured out how to reflect that in LP, and then committed and pushed a new change set. AudioSource and Receiver now have an index attribute, to distinguish between different deck sources or vinyl control receivers. This became necessary because I changed the Audio{Source,Receiver}Type enums to not list discrete decks or vinyl control inputs as that approach will likely prove unextendable (not a real word) in the future, this mainly caught my eye because the number of CSAMPLE*'s being allocated at runtime were hard-set to the number of source types (not good when you have a variable number of sources, and one of my goals with all this is to keep >2 sources in mind, especially since another GSOC student's project uses them.
• Will (tentatively, haven't thought all the way through this one) be using a QHash (hash table-based dictionary/mapping) to implement the assocation of Audio{Source,Receiver}'s to buffers; that's committed as well.
• Committed a couple of static methods that generate an identifying QString from an (new-style) Audio{S,R} (shorthand for Audio{Source,Receiver} I'll continue to use if I remember it as typing all that is a pita). These will probably get moved into their respective structs, which will likely become classes (not that there's any real difference in cpp, but whatever) with ctor's instead of public fields (because I want to make sure Audio{S,R}.index is zero if the type doesn't lend itself to indexing, like SOURCE_MASTER) and some other methods: operator== for QHash (it needs that, plus it needs qHash(AudioSource) and qHash(AudioReceiver) globals defined to generate hashes), and a method to detect whether or not one source's or receiver's channel selection clashes with another (bad!).
• This isn't really worth mentioning as it's said plainly enough in the commit log but I'll paste it: Made the array of VinylControlProxy's a QList, so the number of instances isn't hardcoded. (end paste) It was previously hard-coded to 2, and there shouldn't be any appreciable loss in speed for the [hopefully later realized] gain.
• What's implemented in the commit is not nearly what's in my head or even coded in files elsewhere on my hard-drive, as I tried to convey with all this senseless typing ;)
• Commit at http://bazaar.launchpad.net/~bkgood/mixxx/features_external_mixer/revision/2413
• 20100612
• Commit http://bazaar.launchpad.net/~bkgood/mixxx/features_external_mixer/revision/2414

    * Moved AudioSource and AudioReceiver to audiopath.{h,cpp}
    * Refactored the classes -- instead of having channelBase and channels as members, they now have a member object of type ChannelGroup which has this data, and also includes methods for comparing channel groups to make sure they don't clash (the method implementation in this commit is actually poorly thought out, but an updated one is in a subsequent commit). AudioSource and AudioReceiver also now have a base class AudioPath which basically just has a channel group as the type member is specific to the source or receiver distinction (**note to self: move index data member to audiopath as protected**)
* Commit http://bazaar.launchpad.net/~bkgood/mixxx/features_external_mixer/revision/2415
    * Added qHash fuctions (and related methods to classes) to allow QHash to get hash values for AudioSources and Receivers. Given that qHash must return an unsigned int, and working under the assumptions that an int is 4 bytes and the four unsigned int values which make up an AudioSource or receiver will normally be small and therefore easily representable using 4 1-byte values. If an int is only 2 bytes, some precision will be lost, but QHash can deal with this, with minor slowdowns.
    * ChannelGroup::clashesWith is now fixed from revision 2414.
* Commit http://bazaar.launchpad.net/~bkgood/mixxx/features_external_mixer/revision/2416
    * Gave EngineMaster a getDeckBuffer method to get the unmixed buffers. Required hack until hydra is merged.

Specification

Use cases

• DJ Joe doesn't like lugging around crates of vinyl or CDs, but enjoys using his vinyl turntables (or DJ CD players) and mixer. DVS mode will allow him to control Mixxx's playback with vinyl control, and then mix the outputs with his own mixer, his his own effect modules, etc.

Design

Currently, the sample buffers are only accessible to EngineMaster. These will be exposed via accessor methods so that SoundManager can choose between the master and headphone mixes or the individual samples. Changes made will be conscious of eventual merge of n-decks branch, i.e. will want to look at exposing the various decks using a CSAMPLE* EngineMaster::getDeckByIndex(uint index); method in place of

CSAMPLE* EngineMaster::getDeck1();
CSAMPLE* EngineMaster::getDeck2(); // etc

There is, of course, an upper-bound given the finite nature of PCI slots, USB ports, and sound cards, but a DJ could easily use four decks (8 channels) of output (of course, there's currently only 2 inputs of vinyl control... not sure if n-decks extends that).

• This is all done by hydra, once it gets merged it's all golden :)

The other major aspect of the mode is UI, preferences will have to be extended to support routing audio to an external instead of using an internal mixer. This will likely include a bit of refactoring.

Work breakdown structure

• To be completed :) See Work_breakdown_structure
• Not really proper, but here's my version (in numbered steps!) (aka milestones):

1. External mixer (in lp:~bkgood/mixxx/features_external_mixer). ONLY concerned with taking deck samples and outputting them to two different channel pairs (same or different audio devices). EngineMaster will likely still apply the XF and produce headphone and master buffers and do all the various things it does.
   1. Implement changes to preferences dialog. It will be easier to work from the top down -- make the changes, record the changes with soundmanger/engine, etc. Do this like the current code does, OR (happier version) change current code to not apply until user clicks OK. Committing pref changes after a window destroy (aka X button) or user cancel makes UI gods unhappy. As discussed with Albert, this may be an opportunity to do a bit of refactoring of the audio-related (hell, everything in mixxx is audio-related, but I know what I mean) preferences. May be a required opportunity as prefs dialog can't get any bigger than the screen of a netbook.
   2. Engine details. Make sample buffers accessible. Other stuff, as it makes itself known. I don't foresee this to be complicated, the enginemaster code is relatively short, sweet and modular.
   3. SOUNDMANAGER! This will be the loads-of-fun part. Will probably require non-trivial refactoring of soundmanager (blah). This was the step I started on when I realized I'd rather start on the UI part, even if Qt Designer is a total bitch.
2. DVS mode proper (whatever the hell that is):

• Make connected to external mixer bit.
• Vinyl control UI could use work. Random checkbox menu item is random, vinyl control ought to be enabled in prefs.
• Be able to turn off EQs, faders, FX, gains, ...
• Skin tom foolery. May want to wait until inevitable skin refactor. Could take a couple of approaches in the meanwhile:

    * Make things that don't matter disappear. Of course, since everything is positioned absolutely, no screen real estate is gained, but at least there aren't useless widgets lying around -- just useless space.
    * Load another skin. Either make it user-selectable, or just load CURRENTSKIN-dvs (hacky but user-friendly with a bit of skin hacking on our end).
* Would *love* to have a single timecode deck option.
- Single vinyl deck
* See [[single-deck_vinyl_control]]. Has implementation suggestions.
- Microphone input
* For voice-over. Really has nothing to do with DVS since any mixer worth its salt will have a mic in but mixxx needs it.

Notes

• DVS mode will probably have negligible effect on CPU usage. Some profiling I did [http://article.gmane.org/gmane.comp.multimedia.mixxx.devel/2998] seems to indicate the enginemaster stuff (which includes all the audio processing) isn't actually all that expensive.
• Ok, the hifi eq's can eat CPU really well. May be a bug in eq though, as it stutters real bad on my system without ever peaking out my CPU.
• revamped_control_system has an excellent review of the controlobject system.

Preferences

• Currently there's a bunch of different stuff under "Sound Hardware":
• Audio Output

    * Master device/channel pair
    * Headphone device/channel pair
    * Sample rate
    * Sound API
* Pitch Behaviour
    * Vinyl emulation or PITS (and disclaimer about PITS)
* Latency
    * Slider -- dynamic. Might be a nicer solution than a slider., as the current one is very jerky so doesn't really represent the continuum a slider implies. Might be nicer to use a combobox since it seems to be a pretty discrete set of values.