DRM vs. analogue transmissions of All India Radio

The other day, while scanning the bands, I heard a hum on 7550 kHz that was characteristic of DRM. I fired up my NewStar DR111 (which hardly gets any use nowadays, after Radio Exterior de España ceased their digital transmissions) and tuned in to the frequency. The receiver locked onto the signal immediately and "REGULAR SERVICE" came up on the text display. I had tuned into the General Overseas Service of All India Radio. The signal was very stable without any dropouts, but the sound quality was terrible. I switched back to my Lowe HF-150 and adjusted it to the first analogue frequency of All India Radio I could remember, which happened to be 9445 kHz. I kept switching back and forth between the audio outputs of the two radio receivers and could not believe the difference in quality - the analogue signal was so much better! Below is a recording of this difference: the first 30 seconds are the DRM transmission, and the rest is the analogue signal.

I then recalled that I had encountered this discrepancy in the quality of AIR's transmissions once before and tweeted about it. Below are the tweets comparing the DRM transmission on 7550 kHz to the analogue signal on 11670 kHz back in May 2015.

@K7al_L3afta @SWLingDotCom @swlbcl the music sounds abhorrent! Compare this: pic.twitter.com/S6zBepszQL

— London Shortwave (@LondonShortwave) May 12, 2015

@K7al_L3afta @SWLingDotCom @swlbcl with this! pic.twitter.com/X3lT5z9qla

— London Shortwave (@LondonShortwave) May 12, 2015

The reason this surprised me so much was that All India Radio didn't always sound so bad in DRM. Here's a recording I made in October 2013 on a Morphy Richards DRM receiver:

It turns out that the reason for this drastic change is the fact that All India Radio now combine two audio streams in one DRM feed (one stream per language service), as noted by Andrea Borgnino on Twitter:

Perfect DRM from AIR India (25 db !!) in Roma with 2 audio channel @SWLingDotCom @LondonShortwave @Station_B_B pic.twitter.com/XoNyFhjiVc

— Andrea Borgnino (@aborgnino) December 5, 2015

It's a very cool feature of the DRM standard but unfortunately leaves around 9kbps of bandwidth per stream, resulting in rather terrible audio. The ironic part is that I don't even know how to select the second stream on my DR111 unit and would have to use one of my SDRs together with Dream to take advantage of it. I hope AIR revert to better quality DRM feeds in the future.