I believe you must not be knowing that most of the transponders being used by Airtel Digital TV r of 54 Mhz capacity while Tata Play uses transponders which have capacity load of 36 Mhz. So Airtel Digital TV has 50% additional bandwidth on hand in each transponder compared to Tata Play hence Airtel can a accommodate more channels per transponder.
Secondly one cannot randomly state that this transponder holds n number of channels so there is more compression applied. Each channel run by a broadcaster on C Band is either available in MPEG 2 or MPEG 4 compression technology, so say A channel is running on MPEG 2 on C band and at bi rate of 3.6 Mbps then an DTH operator which is running channels in MPEG 4 compression can uplink this channel on their satellite with bit rate of just 1.8 Mbps maintaining same audio/video quality. Hence when u say an Airtel Digital TV transponder is holding 55 channels it may very well be running all channels in excellent quality.
Another thing is none of the operators provide u any channel in quality as originally available from broadcaster end on C band, they do compress channels by some bit to optimise the use of available bandwidth on their transponders. These days all operators even use encoders to minimise bandwidth needed for running various channels without adversely effecting quality of transmission.
Lastly as i have noticed and even interacted with tech team earlier on about this, Tata Play uses Deblocking technology for video transmission of channels on their platform which smoothens out edges caused by pixels which form a moving picture hence u may at first glance feel that picture quality is good but from sharpness point of view it would be rather on the lower side. On Airtel Digital TV both SD, HD channels r lot more clear, sharper, vibrant.....audio quality on Airtel Digital TV too is extremely good