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Posted by the god Dentin on Jul 06, 2012.

MSP Protocol Used by Alter Aeon

Alter Aeon uses a fairly minimal MSP implementation, consisting primarily of the "off" keyword, the "U=" option, the "V=" option, and a new, nonstandard "R=" option.

Alter Aeon's MSP is intended to be used in download mode, and soundpacks are generally not provided as a unified download. If your client doesn't support the "U=" option, you may run into issues.

When MSP mode is enabled, a "U=" download path is always immediately sent. For example:

!!SOUND(Off U=http://www.alteraeon.com/soundpack/wav_v1/ X=2.0)

(Note that the "Off U=" startup sequence may occur at any time, and clients should handle repeats transparently.)

This startup sequence sets the default sound url and gives the MSP version number. The X tag is the version of MSP supported. Version 2.0 and higher implies that the server supports the "!!SOUNDCTL()" negotiation command.

When a sound is to be played out, a minimal MSP string is sent, with an extra, non-standard "R=" option. If your client does not support the "R=" option, the option should be silently discarded. If your client does not silently discard unknown options, it is broken. (Seriously. Get a new client.)

The original Zuggsoft MSP spec can be found at:

http://www.zuggsoft.com/zmud/msp.htm

While this is the reference specification used by many clients, it should be noted that the spec is extremely old, limited, and has a number of unspecified cases. This document attempts to address some of those issues, as well as more clearly define Alter Aeon's usage of MSP.

Manual Enable of MSP Mode

The commands 'set msp on' and 'set msp off' can be used to manually enable or disable MSP mode on Alter Aeon.

Note that the 'set msp' commands also work at the main login menu. Prior to selecting '1' to enter the game, you may 'set msp on' or 'set msp off' to change MSP mode. This allows MSP to be properly configured prior to login.

Automatic Enable of MSP Mode

On initial connect the server will send the Telnet sequence:

IAC WILL TELNET_MSP

Where TELNET_MSP has the value 90, as per the Zuggsoft MSP spec.

At any time, clients may send:

IAC DO TELNET_MSP
IAC DONT TELNET_MSP

to enable or disable MSP mode.

Non-Standard MSP Extension - Audio Versioning

The "R=" option is used by Alter Aeon to indicate a sound version with each audio file. This is a non-standard extension, and is probably not supported by most mainline clients. Clients shall silently discard this option if they do not understand it.

Client Usage: Clients shall cache downloaded sounds, and keep version information for each sound. If the version found in an "R=" string does not match exactly the cached version, the sound shall be re-downloaded. The version information need not be numeric, and should be matched exactly.

Examples:

!!SOUND(spell/dispel.wav R=5)
!!SOUND(spell/chimes.wav R=6)
!!SOUND(spell/sanctuary.wav R=19)

Implementation note - if a sound is deemed out of date, it is acceptable (but not required) to play out the cached/out of date version of the sound, and start the update download in the background. Downloads take time to complete, and this allows at least some audio to be played out while the updated version is in transit.

Implementation note - if clients wish to allow user overloaded sounds, they may ignore the version information and download process. We recommend that the server specified, downloaded audio cache be kept completely separate from the user-specified audio, with the user-specified directory structure matching the server specified directory structure.

This allows the client to search the user-specified directories for the custom audio first, with the server directories as a fallback. It preserves the integrity and versioning of the server sounds, and allows the user to easily add, remove, or change custom sounds in the user specified directories.

MSP Implementation Note - Audio Preloading

The "V=" option with an argument of '0' is defined as 'play out the sound at zero volume'. Since no audio is played out, this can be used as a 'preloader' to fetch missing data prior to when it is needed.

For this reason, the "V=0" option is used by Alter Aeon to indicate audio preloading. When the "V=0" option is present, clients should search the cache and verify that the sound is present. If the sound is not present, or if the version information does not match, the sound should be re-downloaded for future playout.

Remember that the "V=0" string indicates zero volume. No audio shall be played out, even though downloading may be triggered.

Examples:

!!SOUND(move/step2.wav V=0 R=19)
!!SOUND(misc/muchbetterat.wav V=0 R=19)
!!SOUND(misc/email.wav V=0 R=25)

Non-Standard MSP Extension - Simultaneous Audio

Alter Aeon does not use the "P=" option for determining sound priority. If the server sends a sound, it expects that sound to be played out, simultaneously with other sounds (or with other copies of the same sound) if necessary.

It is recommended that MSP implementors only use the "P=" option to interrupt playout if the option is explicitly sent by the server, for compatibility with older servers which may use the tag to interrupt long-playing audio clips. If the "P=" tag is missing, simultaneous audio clips should be played simultaneously and mixed prior to playout. (See the implementation note below for advice on mixing and compression.)

Example:

A player walks into a room with two aggro mobs. The following three sound commands are sent by the server in extremely quick succession:

!!SOUND(move/gravel3.wav R=19)
!!SOUND(combat/hit2.wav R=19)
!!SOUND(combat/hit2.wav R=19)

The client should play out all three sounds simultaneously, summing them together for final output. Note that the hit2.wav sound should be played out twice.

Implementation Note - because a single sound may be played out repeatedly in quick succession, it is recommended that the client limit the number of times that a single sound can be played simultaneously to 3. This can occur in extremely large groups, or with large notification blocks, where the notification sound is played once for every line.

In the above example, a double playout of hit2.wav is fine, but if there were four attempted simultaneous playouts of hit2.wav, the sound should only be played three times. Further attempts to play hit2.wav should be ignored until previous playout of hit2.wav was completed.

Most Alter Aeon sounds are short and the audio field is sparse, so this condition should trigger rarely, and will likely not be noticed by most players.

MSP Implementation Note - Audio Summing and Compression

It is recommended that simultaneously played audio shall be linearly summed, with overflow compression and soft clipping at extreme levels.

The official Alter Aeon client uses a highly compressed soft clipping scheme for summed audio that amounts of a six part piecewise linear function. This function is fairly easy and quick to implement in software, and should cause no performance issues on modern machines, even if written in normal unoptimized C code.

Input range 0 to 16383
Output range 0 to 16383 - compression factor 1.0

Input range 16384 to 32767
Output range 16384 to 20479 - compression factor 4.0

Input range 32768 to 65535
Output range 20480 to 24575 - compression factor 8.0

Input range 65536 to 131071
Output range 24576 to 28671 - compression factor 16.0

Input range 131072 to 262143
Output range 28672 to 32767 - compression factor 32.0

Input range 262144 and up
Output range 32767 - compression factor infinity

This scheme gives a maximum compression level of 18 db should a large number of very loud sounds be played out simultaneously, while limiting crackling and other ugly sounding distortion typically caused by hard clipping.

MSP Implementation Note - Line Mode and Spoofing

Care should be taken by the server to make !!SOUND() data unspoofable. Server side filtering is recommended so that the "!!SOUND(" string cannot be sent except by the MSP handling code in the server core.

The Alter Aeon implementation will always put an MSP !!SOUND() tag on its own line. The data will not be interleaved in other data. The Alter Aeon implementation cannot be spoofed by players due to server side filtering.

MSP Implementation Note - Unknown Options

MSP clients should ignore unknown options found in the MSP string. Future updates to this document may rely upon older clients ignoring new options.



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