This appendix provides a reference for all RealAudio codecs used by Helix Producer, broken down into separate tables for voice, mono music, and stereo music codecs.
The following are descriptions of the different columns within the codec reference tables. Each column
This column shows the name of the codec as it appears in the Audience Templates dialog. The name gives you vital information about the codec: the bit rate for the codec, the type of audio the codec is suited for, and if the codec is a "high response" codec.
High response codecs cover high frequency sounds, such as a flute, than normal codecs at the same bit rate. But they don't cover low frequencies, such as a bass drum, as well as normal codecs.
The Sampling Rate column is the codec's optimum sample rate. Using a codec's optimum sampling rate in your audio source file ensures that the audio stays synchronized with other media and prevents pitch shifting during audio resampling. Audio quality degrades if you use lower than the optimum sampling rate.
If you use a higher sampling rate when creating the source audio, it is best to use a multiple of the optimum rate. If the optimum rate is 8 kHz, for example, use a higher rate of 16 kHz or 32 kHz. When in doubt, use a CD-quality sampling rate of 44.1 kHz for your source audio.
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For More Information: The audio preparation chapter of RealNetworks Production Guide has an expanded list that covers all RealAudio codecs, including obsolete codecs no longer used by Helix Producer. |
Helix Producer uses a voice codec when you encode a voice-only or voice-with- music clip. The lowest-speed voice codec normally used with RealAudio is 16 kbps. The lower-speed codecs can be used as "duress" streams in SureStream clips. They are also used to encode soundtracks for low-bandwidth RealVideo clips.
| RealAudio Codec | Sampling Rate |
|---|---|
| 5 kbps Voice | 8 kHz |
| 6.5 kbps Voice | 8 kHz |
| 8.5 kbps Voice | 8 kHz |
| 16 kbps Voice | 16 kHz |
| 32 kbps Voice | 22.05 kHz |
| 64 kbps Voice | 44.1 kHz |
Helix Producer contains an array of RealAudio codecs that are optimized for stereo music. These RealAudio codecs allow you to stream high-quality stereo music at a wide range of bit rates.
The biggest difference between stereo and mono music is that mono music has one channel that gets sent to both speakers. Stereo music has separate channels for the left and the right. That means a stereo version of an audio clip needs to hold more data than a mono version. Yet Helix Producer can make stereo and mono clips for the same bandwidth. How can it do this?
The answer lies in the codecs' frequency responses. Remember, RealAudio is a lossy compression scheme that throws out data. The stereo codec squeezes both channels down to the same size as the mono codec by throwing out more data for each channel. This makes its frequency response lower. To put it simply, a stereo clip doesn't represent all frequencies as accurately as a mono clip. You'll hear the two channels, but they may not sound as crisp as a mono channel. This means that a mono codec may be a better choice for low bit rates.
Stereo music steps into the limelight at higher bandwidths. The higher the streaming speed, the better the stereo frequency response. The stereo music codecs were specifically created to create CD-quality stereo music for broadband and portable music users
The 20 kbps, 32 kbps, and 44 kbps music codecs come in two flavors. Helix Producer by default uses the "high response" versions, which are the better codecs for most situations. But you can also use the "normal response" versions.
The high response codecs cover a larger frequency spectrum than the normal response versions. Sometimes, the high response version has twice the range as the normal codec. This means it provides crisper sound and is better at capturing high frequencies. With symphonic music, for example, the high response codec gets more of the flute and piccolo. It can produce more distortion than the normal response codec with voices and loud sounds such as drums, though.
If you're encoding music with a diverse range of frequencies, stick with the high response codecs. If you notice distortion, compare your results with a clip that uses the normal response codecs. The best tool for determining which codec to use is your ear. Listen carefully for minute differences in how the clip sounds. It also helps to have other people listen. Our own ears have different frequency responses, too.
The slowest stereo codec is 12 kbps. Stereo codecs don't go lower than that because they would not have enough frequency response for adequate sound. In the following table, the RealAudio 8 codecs are marked with RA8.
At 44 kbps, 64 kbps, 96 kbps, 132 kbps, 146 kbps, 176 kbps, 264 kbps, and 352 kbps bit rates, you have the choice to use RealAudio Surround codecs. These codecs preserve the matrixed multi-channel surround audio in conventional "surround sound" audio content. Sources of such content are created using Dolby Pro Logic or Circle Surround encoders, and these sources are prevalent on DVDs and television broadcasts.
Surround audio can consist of four sound channels (left, right, left surround, and right surround) or 5.1 channels (additional subwoofer and center) that are mixed into the two conventional left and right stereo channels. Thus, no special cables are needed to encode RealAudio Surround, as long as your audio source is conventional "surround sound" content. While you can use non- digital sources, the best results are achieved from a digital source.
You should only choose the surround versions of the stereo RealAudio codecs if you know that there is surround information in the audio source. The main reason to make sure you have the correct source is that the surround codecs are not as efficient when encoding non-surround audio.
To play back a RealMedia file encoded with RealAudio Surround, you will need to use to an A/V receiver equipped with surround sound decoding that is connected to the surround channels and an optional subwoofer. Your receiver's manual will contain information on how to properly connect surround speakers. Alternatively, some computer speakers (such as Creative Labs Inspire™ 5.1 Digital (5700) speaker system) will play "surround sound" audio.
As with the voice codecs, the lowest-speed mono music codec normally used with RealAudio is 16 kbps. The lower-speed codecs can be used as "duress" streams in SureStream clips, and to encode soundtracks for low-bandwidth RealVideo clips. When there are two versions of a codec, Helix Producer uses the high response version by default.
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