code: func (h FrameHeader) BitRate() FrameBitRate {
  	if len(h) < 4 {
  		return 0
  	}
  	bitrateIdx := (h[2] >> 4) & 0x0F
  	if bitrateIdx == 0x0F {
  		return ErrInvalidBitrate
  	}
  	br := bitrates[h.Version()][h.Layer()][bitrateIdx] * 1000
  	if br == 0 {
  		return ErrInvalidBitrate
  	}
  	return FrameBitRate(br)
  }
docs: // BitRate returns the calculated bit rate from the header