The small but intrepid group of JACK 1 developers announce the release today of JACK 0.124.0, the latest version of JACK 1. This release features 3 major new features that are designed to make JACK easier to use and better in several ways, along with a large set of minor bug fixes and improvements. You can fetch the tarball release here. It has been tagged in the JACK 1 git repository as "0.124.0".

One of the "minor" bug fixes will make a huge difference for some users: if a particular JACK client crashes only the misbehaving client will be "zombified" - other clients will continue functioning normally (after a short delay). If you are developing JACK applications, this can make a huge difference to your day-to-day experience of JACK 1.

The Metadata API

It is now possible to define arbitrary metadata to be associated with any JACK port or JACK client. This API lays the groundwork for more useful naming of ports and clients (particularly those associated with hardware), as well as many other features that developers have talked about in a JACK context for several years. The full docs on the API can be found here and there is also a man page for jack_property, which is the primary command line tool for setting, removing and changing metadata. A short tutorial on using the Metadata API is available.

Builtin MIDI hardware I/O (on Linux)

For years, we have known that the performance of the -X seq and -X raw options to the ALSA backend have left much to be desired. For that reason, most users have been instructed to use a2jmidid (typically with the -e flag to tell it use external (hardware) MIDI devices). Although a2jmidid provided good MIDI I/O, it has two problems:

1. Requires starting an addition program in order to get MIDI I/O via hardware working
2. Adds latency to the MIDI I/O "through" or "monitor" signal pathway

This release of JACK 1 sees the transformation of a2jmidid into a "slave" driver that can be started as a part of JACK, much like the old -X seq and -X raw options. This not only removes the need to start a separate program, but also reduces the "through" or "monitoring" latency of MIDI I/O by 1 JACK period.

The canonical way to invoke this new internal MIDI driver is with -X alsa_midi, which will create JACK ports for all MIDI devices (and other MIDI applications) known to ALSA at the time that JACK starts up. Note that the new internal driver does not currently notice the arrival or departure of MIDI hardware or applications, but future improvements will correct this.

In addition, the old -X seq and -X raw implementations have been removed. However, using -X seq will be interpreted as a request to use the new internal MIDI driver, and so is formally equivalent to using -X alsa_midi. Note that -X seq is an argument to the ALSA backend, and so comes after the backend specification; by contrast, -X alsa_midi is a server argument, and must come before the backend specification.

It is worth noting that the server argument -X alsa_midi can be used with any Linux-based backend (ALSA, OSS, Dummy, Netjack) to get the ALSA MIDI bridge functional.

Builtin Internal Client for using additional ALSA devices

Older versions of JACK have come with the tools alsa_in and alsa_out which can be used to add additional ALSA supported audio devices to a running JACK setup. These tools work quite well, but:

1. Fons Adriennsen developed significantly better versions known as zita-a2j and zita-j2a that use his own resampler.
2. these tools are (like the rest of the JACK tools) separate programs that the user must start up after the server is running

To address these two points, the zita-a2j/zita-j2a code has been merged into the tools part of JACK as a pair of internal clients ("zalsa_in" and "zalsa_out") that can be specified on the command line. So for example, this JACK command line:

	jackd ... -I "USB Mic:zalsa_in/-dhw:USB,-r44100,-n2,-p512" ...

will create an internal client called "USB Mic" that represents the ALSA device "hw:USB" using the ZITA alsa/jack bridge code, running at 44100kHz, with 2 periods of size 512. If you do not specify the period size or sample rate, then the values in use by the backend will be adopted. Audio being captured or played back via devices added to JACK in this way will be resampled with substantial accuracy and quality.

Note that the -I argument has been present in JACK 2 for sometime, but the new JACK1 implementation extends the semantics of its argument by allowing the user to specify a client name as well as arguments for the internal client code.

Both internal clients (zalsa_in and zalsa_out) will accept the -h argument to show their possible arguments: jackd -I zalsa_in/-h -d alsa (the final -d alsa is required by JACK but not used in this case). The arguments will match those of the standalone zita-a2j and zita-j2a (external) clients.

Since this new "multiple device" capability is implemented using JACK internal clients, you can remove a given device by just unloading the client using the long-existing but rarely used jack_unload command. Similarly, if you wanted to add a device to a running server after it has started, jack_load can be used for that.

It is worth noting that these internal clients can be used with any Linux-based backend (ALSA, FFADO, OSS, Dummy, Netjack) to add additional ALSA-support devices to an instance of JACK.

Syntactic Sugar Version

To make the addition of extra audio devices to an instance of JACK even easier, JACK 1 also features the new -A option which takes almost nothing more than an ALSA card name (e.g. "SB", "DSP", "Codec", "HDMI" and many others) and makes the device available for playback or capture or both. Adding %p or %c to the card name limits the direction of audio flow.

This option does nothing more than translate its argument into an equivalent use of -I, so for example:

	-A SB

is equivalent to using both of these arguments:

	-I SB/zalsa_in/-dhw:SB
	-I SB/zalsa_out/-dhw:SB

As with -I, the -A argument may be used multiple times to add as many devices as you want. -A is simple but not very powerful - if you need to provide more options (e.g. to adjust I/O latency values), then you must use the -I argument instead.

Finally, note that these internal clients are theoretically compatible with JACK 2, and should work with that implementation of JACK once JACK 2 has a working internal client loading mechanism restored.

Complete Change List for 0.124.0

Major New Features

• add the new metadata API, allowing arbitrary metadata to be associated with JACK ports and clients
• merge a2jmidid code into JACK as (slave) driver.
• add zita-a2j as internal clients

Notable Source Code Rearrangements

• Use separated JACK public headers and tools repositories, in an attempt to share these with JACK 2
• JACK 1 now requires the Berkeley DB library to build (this is used inside the metadata API implementation)
• The new ZITA-based internal clients are optional - to build them you will need Fons Adriennsen's ALSA PCMI and ZITA resampler libraries available.

Bug Fixes, Updates and Other Enhancements

• Install jslist.h with JACK headers since it is used by control.h
• clarify use of alsa_in/alsa_out's -q parameter
• check for engine->driver before delivery latency events to its internal client
• the default error callback no longer calls DEBUG, which breaks if _jack_get_microseconds has not been set
• remove all references to MMX/MME (which was never used within JACK)
• remove --enable-ensure-mlock configure option
• deprecate jack_thread_wait() as done in JACK2
• clean up client code to fix inconsistent handling of the return value from the process callback
• improve configure.ac for use on OS X
• redesign thread calls to make them work correctly on OS X
• remove port.g from documentation since it is not a public header
• remove timestamps code, since it was no longer used or useful.
• fix detection of libsamplerate
• support setbufsize now available in firewire driver
• improved jack_get_cycle_times() and improved DLL implementation (particularly after freewheeling)
• compile with FFADO versions < 9 once more
• update FFADO latencies after setbufsize
• use max (JACK_CLIENT_EVENT_TIMEOUT,command-line-client-timeout) for poll on event handling when in non-RT mode
• tidy up error messages when something goes wrong with client event handling
• add a latency callback to the ALSA backend, and ensure that the backend's latency callback, if it exists, is called after a buffer size change
• provide FFADO/firewire backend with its own latency callback
• adjust driver/backend port latencies for both playback and capture ports in both directions
• handle latency callbacks before graph reorder callbacks, as per a recent (correct) change in JACK 2
• handled failed malloc caused limits.conf being changed while jackd is running
• Correctly recognize ALSA subdevices when symbolic names like hw:Live are used.
• If an attempt to reconfigure the ALSA backend fails, it will fall back to its previous successful configuration.
• remove watchdog thread from engine/jackd, since Linux no longer allows SCHED_{FIFO,RR} tasks to take over the cpu unless the user misconfigures a very obscure part of their system configuration
• fix delivery of buffer size changes to internal clients
• fix incorrect memset() of an event in latency handling
• fix arguably incorrect marking of client with errors after process cycle fails to return to server
• use a default size of 2048 byte for MIDI port buffers, like JACK 2, to avoid problems with MIDI delivery under fairly common circumstances
• add -I option to command line to allow loading internal clients (consistent with JACK 2), though with more complex argument style to -I to permit client names and arguments

Authors

Jonathan Woithe, Adrian Knoth, Fons Adriennsen, Nedko Arnaudov, Torben Hohn, Paul Davis.

Thanks to David Robillard, Jonathan Liles and Filipe Coehlo for their participation in the design of the metadata API.