Windows Media Encoder organizes audio and video data into data
packets that can be easily and efficiently transported over a
network. When the packets arrive at the client, Windows Media
Player 10 re-assembles the data in the correct order and plays
the audio and video with the correct synchronization. Because the
video buffer is typically larger than the audio buffer, it is
common for the encoder to place the audio data in a packet that
arrives after the corresponding video. When this occurs, the Player
must wait for the audio to arrive before it begins rendering a
stream because video cannot play without the accompanying audio
stream.
Audio that arrives late is typically not a problem if startup
latency is not an issue in the streaming scenario. However, when
Advanced Fast Start is used to minimize latency, it is important
that the audio arrive slightly before the video so the content can
begin playing as soon as possible. To make sure the audio arrives
before the video, you can create an encoder profile in which the
audio and video buffers are the same size. The fixed audio buffer
size is 1450 milliseconds (ms). Therefore, if the profile sets
audio and video buffers to 3000 ms, the audio buffer is 3000 ms
plus the fixed buffer size, or 4450 ms, which effectively places
the audio data before the video data.
To create a profile that includes an audio buffer setting, first
create a custom profile with Windows Media Profile Editor, and then
add the setting by opening and editing the custom profile file with
a text editor, such as Notepad.
To add an audio buffer setting to an encoding profile
On the Start menu, point to All Programs, point
to Windows Media, point to Utilities, and then click
Windows Media Profile Editor.
On the General tab, enter the following information:
Name and Description. Enter text to identify your
custom profile.
Mode and Codec. In Media Types, select the
Audio and Video check boxes. Make sure CBR
(constant bit rate) is selected for audio and video, and Windows
Media Audio 9 and Windows Media Video 9 codecs are
selected.
Select the Allow interlaced processing and Allow
non-square pixel output check boxes (This is not required for
Advanced Fast Start, but is recommended).
Target bit rates. Click Add, and type any number
over 15K. You will adjust the final bit rate on the Bit
Rate tab.
On the Bit Rate tab, enter the following information:
Audio format. Select a compression setting and format.
For example, for high-quality audio, select 64 Kbps, 48 kHz,
stereo CBR.
Video size. Select the Same as video input check
box.
Frame rate. Enter an appropriate frame rate. For highest
quality, the frame rate should be the same as the source, for
example 29.97 for NTSC video.
Key frame interval. Enter the minimum key-frame interval
value of 1.
Video bit rate. Enter an appropriate high-bandwidth
video bit rate, for example 1200 Kbps or 1.2
Mbps.
Buffer size. For unicast delivery with Advanced Fast
Start, enter 3. For multicast, enter the minimum buffer size
of 1.
Video smoothness. Enter a value of 83.
Click Save and Close.
In the Save Profile As dialog box, locate the
Profiles folder (by default, %systemdrive%\Program
Files\Windows Media components\Encoder\Profiles), and enter a file
name for the profile.
Close Windows Media Profile Editor, open a text editor such as
Notepad, and then open the custom profile file.
In the profile text, specify the bufferwindow value as
4450:
bufferwindow="4450"
Save the file.
Note
The audio buffer setting described in this
section is a guideline that works well for many, but not all,
streaming scenarios. For more information, see "Delivering IPTV
with the Windows Media Platform" on the Microsoft Web site.
Note