audio – Page 3 – Engineering Radio

Hearing test

This was in the Wheatstone newsletter a few months ago. NPR has an interesting test to see if one can hear the difference between various quality .mp3 and .wav files. There are six cuts with three versions each; a 128 kbps .mp3, a 256 kbps .mp3, and a .wav file in no particular order.

That NPR article can be found here: How Well Can You Hear Audio Quality?

I listened to all of them and found the 128 kbps .mp3 was pretty easy to pick out. On the newer material, it was sometimes difficult to tell the difference between the .wav file and the 256 kbps .mp3. Keep in mind that most radio stations stream at 64-128 kbps. Online music services like Pandora (64 kbps for free listeners, 192 kbps for subscribers), Spotify (96-160 kbps for free listeners, 320 kbps for subscribers), and Apple (256 kbps for everybody) offer slightly better quality, especially for paid subscribers.

It is too bad one cannot simulate 15 IPS analog tape. I would bet that a well-mastered recording on analog tape would stand out above anything even remotely compressed.

Others have compared streaming audio to analog FM audio and found that fairly high bit rates are needed to make the quality equal: What bitrate is needed to sound like analog FM?

One more thing to keep in mind, HD Radio runs 96-144 kbps on the main channel and 20-60 kbps on the sub-channels.

Cable Porn

On occasion, the company I currently work for does installation work. Thus, I am always keeping my eyes open for new equipment and tools to make that job easier. The cable comb seems like it is just such a thing:

Instructional video from youtube:

Then there is this:

Which is simply amazing. It is described as “1320 Category 6 cables, dressed and terminated.”

Incidentally, there is an entire subreddit: reddit.com/r/cableporn for all those cable geeks that like to look at neat cabling work.

What bitrate is needed to sound like analog FM?

As it turns out, 300 kbp/s or greater. At least in critical listening environments according to the paper titled Perceived Audio Quality of Realistic FM and DAB+ Radio Broadcasting Systems (.pdf) published by the Journal of the Audio Engineering Society. This work was done by a group in Sweden that made various observations with different program material and listening subjects. Each person was given a sample of analog FM audio to listen to, then they listened to various audio selections which were using bit reduction algorithms (AKA CODEC or Compression) and graded each one. The methodology is very thorough and there is little left for subjective interpretation.

In less critical listening environments, bit rates of 160-192 kbp/s will work.

I made a chart and added HD Radio’s proprietary CODEC HDC, which is similar to, but not compatible with AAC:

System	Codec	Bit Rate (kbp/s)
HD Radio FM; HD1 channel*	HDC (similar to AAC)	96 – 144
HD Radio FM; HD2 channel*	HDC	24-48
HD Radio FM; HD3 channel*	HDC	24-48
HD Radio AM*	HDC	20-60
DRM30 (MF-HF)	AAC/HE-AAC	34-72
DRM+ (VHF)	AAC/HE-AAC	700
DAB+	AAC/HE-AAC	32 – 128
DAB	MPEG II, Dolby Digital	192 – 256
Blu-ray	PCM**	≥6 Mbp/s
DVD	PCM, DTS, Dolby Digital	>800
CD-A	PCM	1,411
Web Streaming	MPEG I,II,III, WMA, AAC, etc	32-320, 128 typical
iTunes	AAC	128 – 256
Spotify	Ogg Vorbis	96 – 320
Wimp	AAC/HE-AAC	64 – 256

*Hybrid mode
**PCM: uncompressed data

This is the composite Mean Basic Audio Quality and 95% confidence intervals for the system across all excerpts:

Over the years, we have simply become accustomed to and now accept low-quality audio from mp3 files being played over cheap computer speakers or through cheap ear buds. Does this make it right? In our drive to take something good and make it better, perhaps it should be, you know: Better.

Special thanks to Trevor from Surrey Electronics Limited.

Analog Sauce

A little blast from the past. This was found in a transmitter manual at one of the sites we take care of:

I thought I would scan it and make it available here. As luck would have it, there is also a corresponding piece of equipment to go along with it. I had never seen a “CCA Optomod” (.pdf) before I was working at one of the radio stations in Trenton, Florida. This unit was rescued from under a pile of garbage out in the lawn shed. It was full of mud wasp nests and mouse droppings. Needless to say, it required a bit of TLC to return it to operation. I replaced the electrolytics, cleaned it up, and ran some audio through it. It is probably as good as the day it left the factory. Bob Orban made some really good stuff in his day.

The original Optomod 8000 was an evolutionary design that made FM radio processing what it is today. The idea of combining broadband limiter, AGC and stereo generator in one box was a radical departure from the norm. The audio limiter functioned as a 15 KHz low pass filter and broadband AGC.

Orban Optomod 8000 audio limiter block diagram

The stereo generator used very modest amounts of composite clipping to reduce overshoot and transients. Many people disparage composite clippers. If done correctly, it is transparent to the listener and increases perceived loudness by stripping off modulation product that is non-productive.

Orban Optomod 8000 Stereo Generator block diagram

Some thirty five or so years later, there are still many of these units in service in various stations around the world.