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Pro Excellent voice quality
Since version 1.2.4, Mumble uses an audio codec named Opus that's specifically developed to provide low-latency voice communication over Internet. This means that there is no delay time, while audio still comes out clear.
Pro Voices sound like they come from where the player is in-game
Mumble supports positional audio for over 50 games, including versions of Call of Duty, Battlefield and Counter-Strike. Positional audio allows hearing the audio from the direction of the player in game, making it easier to tell who is saying it and also giving you a rough idea of where they are. You can find a full list of supported games here.
Pro Low latency and resource usage
Mumble is often praised for near-instant communication it provides with little resource usage.
Pro Code available to anyone to inspect or use
Pro Can be used on a gamers preferred platform
Available on Windows, Linux, OS X, Android and iOS which allows for the user to choose what operating system works for them and still be able to use Mumble.
Pro Easily overlay over game
An in-game overlay can be enabled to show users in the channel or users talking. Overlay can be disabled completely or on a per-game basis by going to Settings->Overlay. This allows for users to see their chat but not obstruct the game they are playing.
Pro Good audio normalization
Mumble includes great automatic volume adjustment functionality that can be used to prevent big volume changes while still allowing good communication.
Pro Use of certificates offers more security and easy people management
Mumble uses certificates for user identification. A certificate can be generated by the user himself or issued by a third-party organization with organization certificates having 2 classes. A self-generated certificate is not tied to anything, a Class 1 certificate is tied to an e-mail address and a Class 2 certificate requires proof of identity. Depending on the server a different level of certification might be required.
Pro Free to download and use as a client
Mumble is completely free unless you own a server yourself.
Pro Secure & encrypted
This encryption is mandatory and cannot be disabled. The so-called control channel, which transports your chat messages and other non-time critical information, is encrypted with TLS using 256 bit AES-SHA. The voice channel carrying speech and positional audio is encrypted with OCB-AES 128 bit.
Pro Server works on Raspberry Pi
Mumble server side is easy to setup and works smooth even on Raspberry Pi, which is pretty short on resources.
Pro Cross platform, with mobile apps available
Client works on Windows, Linux, OS X, iOS, Android, and server applications for Windows, Linux.
Con A bit more difficult to set up than similar solutions
The process of generating and/or acquiring certificates can be time-consuming and a bit more complex than setting up a username/password authentication that's found in other clients.
Similarly, setting up a server that includes configuring user permission system requires advanced technical knowledge.