The　level　selection　of　frequency　band　division　structure　relies　on　prior　information　in　most　gram　approaches　that　capture　the　optimal　demodulation　frequency　band　(ODFB).　When　an　improper　level　is　specified　in　these　approaches,　the　fault　characteristic　information　contained　in　the　produced　ODFB　may　be　insufficient.　This　study　proposes　a　unique　approach,　termed　median　line-gram,　to　tackle　the　level　selection　problem.　To　divide　the　frequency　domain　signal　into　a　series　of　demodulation　frequency　bands,　a　spectrum　median-line　segmentation　model　based　on　Akima　interpolation　is　first　created.　The　level　and　boundary　of　the　segmentation　model　can　be　adaptively　identified　by　this　means.　Second,　the　acquired　frequency　bands　are　quantized　using　the　negative　entropy　index,　and　the　ODFB　is　defined　as　the　frequency　band　with　the　largest　value.　Third,　the　envelope　spectrum　is　used　to　determine　the　ODFB　characteristic　frequency　to　pinpoint　the　bearing　fault　location.　Finally,　both　simulation　and　experimental　signal　analysis　are　used　to　demonstrate　the　efficiency　of　the　suggested　method.　Furthermore,　the　proposed　method　extracts　more　defect　feature　information　from　the　ODFB　than　existing　methods.　©　2022　IOP　Publishing　Ltd.