1.修改原理

通过修改或者添加sdp中的字段实现控制码率的功能。

2. 修改代码示范

2.1 python

def setStartBitrate(codec,isVideoCodec:bool,sdp:str,bitrateKbps):
    lines = sdp.split("\r\n")
    rtpmapLineIndex = -1
    sdpFormatUpdated = False
    codecRtpMap = None
    for i,line in enumerate(lines):
        print(line)
        m = re.search("^a=rtpmap:(\\d+) " + codec + "(/\\d+)+[\r]?$",line)
        if m:
            codecRtpMap=m.group(1)
            rtpmapLineIndex=i
            break
    if codecRtpMap==None:
        return sdp
    print(f"Found " + codec + " rtpmap " + codecRtpMap + " at " + lines[rtpmapLineIndex])
    for i,line in enumerate(lines):
        m = re.search("^a=fmtp:" + codecRtpMap + " \\w+=\\d+.*[\r]?$",line)
        if m:
            print("Found " + codec + " " + lines[i])
            if isVideoCodec:
                lines[i] += "; " + "x-google-start-bitrate=" + bitrateKbps
            else:
                lines[i] += "; " + "maxaveragebitrate=" + (bitrateKbps * 1000)
            sdpFormatUpdated = True
            break
    newSdpDescription:str=""
    for i,line in enumerate(lines):
        newSdpDescription+=(line+"\r\n")
        if not sdpFormatUpdated and i==rtpmapLineIndex:
            bitrateSet=""
            if isVideoCodec:
                bitrateSet="a=fmtp:" + codecRtpMap + " x-google-start-bitrate=" + str(bitrateKbps)
            else:
                bitrateSet = "a=fmtp:" + codecRtpMap + " maxaveragebitrate="+ str(bitrateKbps * 1000)
            newSdpDescription+=(bitrateSet+"\r\n")
    return newSdpDescription

2.2 JAVA代码

摘录webrtc的webrtcapp示例

private static final String VIDEO_CODEC_PARAM_START_BITRATE = "x-google-start-bitrate";
	private static final String AUDIO_CODEC_PARAM_BITRATE = "maxaveragebitrate";
  @SuppressWarnings("StringSplitter")
  private static String setStartBitrate(
      String codec, boolean isVideoCodec, String sdp, int bitrateKbps) {
    String[] lines = sdp.split("\r\n");
    int rtpmapLineIndex = -1;
    boolean sdpFormatUpdated = false;
    String codecRtpMap = null;
    // Search for codec rtpmap in format
    // a=rtpmap:<payload type> <encoding name>/<clock rate> [/<encoding parameters>]
    String regex = "^a=rtpmap:(\\d+) " + codec + "(/\\d+)+[\r]?$";
    Pattern codecPattern = Pattern.compile(regex);
    for (int i = 0; i < lines.length; i++) {
      Matcher codecMatcher = codecPattern.matcher(lines[i]);
      if (codecMatcher.matches()) {
        codecRtpMap = codecMatcher.group(1);
        rtpmapLineIndex = i;
        break;
      }
    }
    if (codecRtpMap == null) {
      Log.w(TAG, "No rtpmap for " + codec + " codec");
      return sdp;
    }
    Log.d(TAG, "Found " + codec + " rtpmap " + codecRtpMap + " at " + lines[rtpmapLineIndex]);

    // Check if a=fmtp string already exist in remote SDP for this codec and
    // update it with new bitrate parameter.
    regex = "^a=fmtp:" + codecRtpMap + " \\w+=\\d+.*[\r]?$";
    codecPattern = Pattern.compile(regex);
    for (int i = 0; i < lines.length; i++) {
      Matcher codecMatcher = codecPattern.matcher(lines[i]);
      if (codecMatcher.matches()) {
        Log.d(TAG, "Found " + codec + " " + lines[i]);
        if (isVideoCodec) {
          lines[i] += "; " + VIDEO_CODEC_PARAM_START_BITRATE + "=" + bitrateKbps;
        } else {
          lines[i] += "; " + AUDIO_CODEC_PARAM_BITRATE + "=" + (bitrateKbps * 1000);
        }
        Log.d(TAG, "Update remote SDP line: " + lines[i]);
        sdpFormatUpdated = true;
        break;
      }
    }

    StringBuilder newSdpDescription = new StringBuilder();
    for (int i = 0; i < lines.length; i++) {
      newSdpDescription.append(lines[i]).append("\r\n");
      // Append new a=fmtp line if no such line exist for a codec.
      if (!sdpFormatUpdated && i == rtpmapLineIndex) {
        String bitrateSet;
        if (isVideoCodec) {
          bitrateSet =
              "a=fmtp:" + codecRtpMap + " " + VIDEO_CODEC_PARAM_START_BITRATE + "=" + bitrateKbps;
        } else {
          bitrateSet = "a=fmtp:" + codecRtpMap + " " + AUDIO_CODEC_PARAM_BITRATE + "="
              + (bitrateKbps * 1000);
        }
        Log.d(TAG, "Add remote SDP line: " + bitrateSet);
        newSdpDescription.append(bitrateSet).append("\r\n");
      }
    }
    return newSdpDescription.toString();
  }