![]() ![]() The packet size limit will automatically result in the appropriate VBR quality setting that can maintain the packet size limit. Figure 2 Illustration of the bit buffering scheme in CBR mode to achieve a perfectly constant transmission rate.įirst appearing in iOS 6.0, applications that need to limit the maximum packet size can force the encoder to maintain that limit. ![]() The transmission rate in this scenario is perfectly constant and the buffer fullness at the encoder and decoder always add up to 100%. ![]() 2 illustrates the bitstream buffering at the encoder and decoder site to smooth out the variable packet sizes of the encoder. The bitstream buffering is used to enable a constant bit rate transmission of variable packet sizes.įig. In this case the end-to-end delay is the codec delay plus the delay due to the bitstream buffering. This mode conforms to the CBR mode as defined in the MPEG-4 standard. In CBR mode, the packet sizes are constrained so that a true constant bit rate can be achieved with a specified end-to-end delay. The packet size can vary significantly depending on the audio content up to 6144 bits per channel. The encoder generates packets of varying size representing constant-size blocks of the input audio signal. While the ABR mode is tailored to control file size in offline applications, the VBR and CBR modes are more appropriate for real time. Real-time Applicationsįor real-time applications such as streaming or communications, the instantaneous packet sizes and temporally varying bit rates can be more important depending on the properties of the transmission channel. The average bit rate in this mode will be at least as high as in ABR mode but it is permitted to increase if the quality would otherwise drop. The VBR_constrained mode provides a compromise between ABR and VBR modes. The target quality can be controlled using VBR mode. The target file size can be controlled in ABR mode, with the bit rate approximation of file size divided by the audio duration. Ultimately, the goal here is to achieve the best audio quality for a given file size, or conversely, to get the smallest file size for a given quality. Hence, for these applications the instantaneous bit rate fluctuation and varying packet sizes are less important. In offline or file-based applications the compressed file size is the most important measure related to the bit rate or quality setting. Figure 1 Illustration of packets representing the audio signal. All control modes will generate packets with varying size as illustrated in Fig. In VBR mode the bit rate will increase when the quality parameter is increased but there is no direct control of the average bit rate. The CBR mode provides the tightest control of the bit rate target, while ABR mode and VBR_constrained mode impose lesser restrictions on the bit rate variation over time. The amount of output data generated by the encoder is commonly specified by the bit rate parameter in bits per second. Back to Top Specifying the amount of output data and target quality ![]()
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