I do not have IC-7400, hi.
The March issue of Japanese CQ Ham Radio Magazine has an article on IC-7400 (aka IC-746PRO outside Japan). It is an interview with lead ICOM engineers of the IC-7400 project. The original article is written in Japanese. Here is my English summary for non-Japanese HAMs.
The front panel of IC-7400 looks like the predecessor, IC-746, but ICOM has named it IC-7400, not IC-746II or IC-746PRO. It is true that the IC-7400 was made to replace IC-746, but IC-7400 also is a little brother of IC-756PROII. The appearance of IC-7400 follows IC-746, but its TX/RX architecture including frequency composition structure is very much close to IC-756PROII. This is why ICOM has given a name with four-digit number.
The development of the IC-7400 DSP circuit was done two years later from that of IC-756PROII DSP. For this reason, the DSP chip deployed in IC-7400 has lower price yet higher performance than the DSP chip used in IC-756PROII. For cost saving, IC-7400 uses 24-bit A/D and D/A converters that have a couple dB higher noise floor than IC-756PROII's. The 'figures' of the converters are inferior to IC-756PROII, but the difference in practical operations is negligible from the viewpoint of the NF in the whole RX circuit.
Some users are disappointed that IC-7400 does not have 430MHz built-in. ICOM actually considered the possibility of 430MHz, but they have given up. The reasons are: (1) the design of the TX power amplifier must be redone, (2) the chassis must be redesigned accordingly, (3) the price would be as high as IC-756PROII, and (4) the demand of 430MHz is not high in the US market compared with the JA market.
The RX front-end band-pass filter unit was redesigned. It employs big inductors and capacitors to prevent the magnetic saturation. In addition, it uses low-distortion switching diodes for the filter selection as IC-756PROII does.
- IC-7400 has Voice Squelch Control (VSC), which opens only for the signal having voice audio.
- IC-7400 has a voice equalizer both that works for both TX and RX.
- The speech compressor works AM/FM as well as SSB. This is a request from 50MHz AM enthusiasts. In AM, it works just like an automatic mic gain.
ICOM is now working on an advanced DSP solution, which will use higher frequency for the DSP input signals. The input frequency of the IC-7400 DSP is 36kHz. Although the cost trade-off must be taken into account, ICOM would like to leverage it up to 100kHz in the near future. The DSP running at higher frequency certainly alleviates the distortion and spurious.
ICOM is very proud of the price-performance of IC-7400; the engineers in the interview said "IC-7400 is ALMOST PERFECT."