How to convert mono audio to stereo audio using Web Audio API - web-audio-api

Question

I am getting a remote media stream in which both audio and video resides. In this stream i am getting mono audio while i need stereo audio.
So how can i convert mono audio to stereo audio using Web Audio API

Answer 1

August 2016 update:
Apparently the behavior of the .connect() call changed; in the past the input/output indices were automatically incremented with each call, but now they just default to 0 - so when unspecified the calls will always connect output 0 to input 0.
If the input stream has two channels, but only one of the channels is used, you'll have to manually route that channel to both the left and right speakers. It is possible to break the two channels in a stereo connection (the connection with the MediaElementSource) up into two separate mono connections by using a ChannelSplitter. Then the left (or right, depending on your usage) channel-connection can easily be routed to the left and right connections of the ChannelMerger (thanks to fan-out support allowing a single output to be connected to multiple different inputs), which'll combine all it's mono input connections back into one stero output connection. The Gain node as shown in the old answer is unnecessary.
These connections can be made by specifiying the correct indices to the connect(AudioNode destination, optional unsigned long output = 0, optional unsigned long input = 0); call, as mentioned above.
//create a source node to capture the audio from your video element
source = context.createMediaElementSource(document.querySelector('video'));
//Create the splitter and the merger
splitter = context.createChannelSplitter();
merger = context.createChannelMerger();
//route the source audio to the splitter. This is a stereo connection.
source.connect(splitter);
//route output 0 (left) from the splitter to input 0 (left) on the merger. This is a mono connection, carrying the left output signal to the left input of the Merger.
splitter.connect(merger, 0, 0);
//route output 0 (left) from the splitter to input 1 (right) on the merger. This is a mono connection as well, carrying the left output signal to the right input of the Merger.
splitter.connect(merger, 0, 1);
//finally, connect the merger to the destination. This is a stereo connection.
merger.connect(context.destination);
And here is what it looks like in a diagram. Remember that the connections between the input and splitter, and the Merger and destination are stereo connections (or more, depending on the configuration - when you have a 2.1, 5.1 or 7.1 set the connection between the merger and destination can contain 3, 6 or 8 channels respectively), whilst the two connections between the Splitter and Merger are mono connections.
+--------------+ +------------+ +-------------------+ +-----------+
| Stereo input |===>| Splitter | | Merger |===>|destination|
+--------------+ | channel0 |--->| channel0 (left) | +-----------+
| channel1 | \ | |
| etc | ->| channel1 (right)|
+------------+ +-------------------+
I am not a hundred percent sure, but this might work with the channel merger node. You just connect a gainnode to both input 1 and 2. (call .connect twice).
Edit (I have time now, so a more complete answer):
Do you really receive one channel audio, as webAudio should mix that automatically, according to this document, which states: For example, if a mono audio stream is connected to a stereo input it should just mix to left and right channels appropriately.. If you receive a stereo stream, where only one channel contains data, you will need to split this up in two channels, and then connect the channel with the audio to both left and right: (working example here)
gain = context.createGain();
splitter = context.createChannelSplitter();
merger = context.createChannelMerger();
merger.connect(context.destination);
source.connect(splitter);
splitter.connect(gain);
gain.connect(merger);
gain.connect(merger);
What happens on the merger and splitter is that when you call .connect, you take the next channel, but you just want to take the first channel, and then split that. So we route that to a gain node, and split it from there:
+----------------+ +----+ +------------------+
+----------+ | Splitter | |gain| | Merger |
|mono input|--->| channel0 |--->| |---->| channel0 (left) | +-----------+
+----------+ | channel1 | | | | |-->|destination|
| etc | | |---->| channel1 (right) | +-----------+
+----------------+ +----+ +------------------+