 Digital television channels can be interleaved and are in a highly compressed format, so that the bandwidth they require varies due to the bitrate provided to each channel; it is more cost efficient to transmit several channels together so that they share the same bandwidth, each channel has carefully constrained bitrates allocated, so that the sum of all channels fill the fixed bandwidth provided.A group of program services transmitted within a particular bandwidth allocation is erroneously known as a multiplex; or the channels may be called subchannels.Sometimes, when analog transmissions are replaced by digital, the fixed bandwidth of one analog broadcast is allocated to the program services; the bandwidth of one analog broadcast is sufficient for several compressed channels.